Interferometer system and method of manufactruing projection optical system using same

ABSTRACT

A method of manufacturing a projection optical system ( 37 ) for projecting a pattern from a reticle to a photosensitive substrate, comprising a surface-shape-measuring step wherein the shape of an optical test surface ( 38 ) of an optical element ( 36 ) which is a component in the projection optical system is measured by causing interference between light from the optical surface ( 38 ) and light from an aspheric reference surface ( 70 ) while the optical test surface ( 38 ) and said reference surface ( 70 ) are held in integral fashion in close mutual proximity. A wavefront-aberration-measuring step is included, wherein the optical element is assembled in the projection optical system and the wavefront aberration of the projection optical system is measured. A surface correction calculation step is also included wherein the amount by which the shape of the optical test surface should be corrected is calculated based on wavefront aberration data obtained at the wavefront-aberration-measuring step and surface shape data obtained from the surface-shape-measuring step. The method also includes a surface shape correction step wherein the shape of the optical test surface is corrected based on calculation performed at the surface correction calculation step. Surface shape measuring interferometer systems and wavefront-aberration-measuring interferometer systems ( 22 J- 22 Q) used in performing the manufacturing method are also disclosed.

FIELD OF THE INVENTION

[0001] The present invention relates to an interferometer system formeasuring the shape of an aspheric surface of an optical element in anoptical system and for measuring the wavefront aberration of such anoptical system, particularly in connection with manufacture of aprojection optical system suited to for use in an exposure apparatusemploying soft-X-ray (EUV) exposure light.

BACKGROUND OF THE INVENTION

[0002] Light of wavelength 193 nm or longer has hitherto been used asthe exposure light in lithographic equipment used when manufacturingsemiconductor devices such as integrated circuits, liquid crystaldisplays, and thin film magnetic heads. The surfaces of lenses used inprojection optical systems of such lithographic equipment are normallyspherical, and the accuracy in the lens shape is 1 to 2 nm RMS (rootmean square).

[0003] With the advance in microminiaturization of the patterns onsemiconductor devices in recent years, there has been a demand forexposure apparatus that use wavelengths shorter than those usedheretofore to achieve even greater microminiaturization. In particular,there has been a demand for the development and manufacture ofprojection exposure apparatus that use soft X-rays of wavelength of 11to 13 nm.

[0004] Lenses (i.e., dioptric optical elements) cannot be used in theEUV wavelength region due to absorption, so catoptric projection opticalsystems (i.e., systems comprising only reflective surfaces) areemployed. In addition, since a reflectance of only about 70% can beexpected from reflective surfaces in the soft X-ray wavelength region,only three to six reflective surfaces can be used in a practicalprojection optical system.

[0005] Accordingly, to make an EUV projection optical systemaberration-free with just a few reflective surfaces, all reflectivesurfaces are made aspheric. Furthermore, in the case of a projectionoptical system having four reflective surfaces, a reflective surfaceshape accuracy of 0.23 nm RMS is required. One method of forming anaspheric surface shape with this accuracy is to measure the actualsurface shape using an interferometer and to use a corrective grindingmachine to grind the surface to the desired shape.

[0006] In a conventional surface-shape-measuring interferometer,measurement repeatability is accurate to 0.3 nm RMS, the absoluteaccuracy for a spherical surface is 1 nm RMS, and the absolute accuracyof an aspheric surface is approximately 10 nm RMS. Therefore, therequired accuracy cannot possibly be satisfied. As a result, aprojection optical system designed to have a desired performance cannotbe manufactured.

[0007] So-called null interferometric measurement using a null(compensating) element has hitherto been conducted for the measurementof aspheric surface shapes. Null lenses that use spherical lensescomprising spherical surfaces, and zone plates wherein annulardiffraction gratings are formed on plane plates have principally beenused as null elements.

[0008]FIG. 1 shows a conventional interferometer system 122 arrangementfor null measurement using a null (compensating) element 132. Theinterferometric measurement described herein is a slightly modifiedversion of a Fizeau interferometric measurement. Namely, a plane wave126 emitted from an interferometric light source 124 is partiallyreflected by a high-precision Fizeau surface 130 formed on a Fizeauplane plate 128. The component of plane wave 126 transmitted throughFizeau surface 130 is converted into measurement wavefront (nullwavefront) 134 by null element 132 and assumes a desired aspheric designshape at a measurement reference position RP, following which it arrivesat a test surface 138 of a test object 136 previously set at thereference position. The light arriving at test surface 138 is reflectedtherefrom and interferes with the light component reflected from Fizeausurface 130, and forms monochromatic interference fringes insideinterferometer system 122. These interference fringes are detected by adetector such as a CCD (not shown). A signal outputted by the detectoris analyzed by an information processing system (not shown) thatprocesses the interferometer information contained in the output signal.Similar measurements can be performed using a Twyman-Greeninterferometer.

[0009] To accurately ascertain the shape of test surface 138, the nullelement 132 must be manufactured with advanced technology since theremust be no error in the null wavefront. Specifically, this means thatthe optical characteristics of the null element 132 must be measuredbeforehand with high precision. Based on these measurements, the shapeof null wavefront 134 is then determined by ray tracing. This results inthe manufacture of null element 132 taking a long time. Consequently,the measurement of the desired aspheric surface takes a long time.

[0010]FIG. 2 shows another example of a conventional Fizeauinterferometer 222. Referring to FIG. 2, laser light from laser 224passes through a lens system 226 to become a collimated light beam of aprescribed diameter and is incident Fizeau plate 228. Rear side 230 ofFizeau plate 228 is accurately ground to a highly flat surface, and thecomponent of the incident light reflected by rear side 230 of Fizeauplate 228 becomes a reference beam having a plane wavefront. Thecomponent of incident light transmitted through a Fizeau plate 228passes through null element 232, where the plane wavefront where theplane wavefront is converted to a desired aspheric wavefront. Theaspheric wavefront is then incident in perpendicular fashion an aspherictest surface 238. The light reflected by test surface 238 returns alongthe original optical path, is superimposed on the reference light beam,reflects off a beam splitting element 256 in lens system 226, and formsinterference fringes on a CCD detector 260. By processing theseinterference fringes by a computer (not shown), the shape error can bemeasured.

[0011] A problem with interferometer 222 is deterioration, in absoluteaccuracy, due to null element 232. A null element comprising a number ofhigh-precision lenses (e.g., lenses 234 and 236) a CGH(computer-generated hologram), or the like is ordinarily used as nullelement 232, and manufacturing errors on the order of 10 nm RMStypically result.

[0012] Since interferometer 222 tends to be affected by vibration andair fluctuations due to the separation of reference surface 230 (i.e.,rear side of Fizeau plate 228) and test surface 238. Repeatability isalso poor, at 0.3 nm RMS. Furthermore, in measuring an aspheric surface,alignment of null element 232 and test surface 238 is critical.Measurement repeatability deteriorates by several nanometers ifalignment accuracy is poor.

SUMMARY OF THE INVENTION

[0013] The present invention relates to an interferometer system formeasuring the shape of an aspheric surface of an optical element in anoptical system and for measuring the wavefront aberration of such anoptical system, particularly in connection with manufacture of aprojection optical system suited to for use in an exposure apparatusemploying soft-X-ray (EUV) exposure light.

[0014] The goal of the present invention is to overcome theabove-described deficiencies in the prior art so as to permit fast andaccurate calibration of a null wavefront corresponding to an asphericsurface accurate to very high dimensional tolerances.

[0015] Another goal of the present invention is to manufacture aprojection optical system having excellent performance.

[0016] Additional goals of the present invention are to provide anaspheric-surface-shape measuring interferometer having goodreproducibility, to measure wavefront aberration with high precision andto permit calibration of an aspheric-surface-shape measuringinterferometer so as to improve absolute accuracy in precision surfacemeasurements.

[0017] Accordingly, a first aspect of the invention is an interferometercapable of measuring a surface shape of a target surface as compared toa reflector standard. The interferometer comprises a light sourcecapable of generating a light beam, and a reference surface arrangeddownstream of the light source for reflecting the light beam so as toform a reference wavefront. The interferometer further includes a nullelement arranged downstream of the reference surface for forming adesired null wavefront from the light beam. The null element is arrangedsuch that the null wavefront is incident the target surface so as toform a measurement wavefront and is also incident the reflector standardwhen the latter is alternately arranged in place of the target surfaceso as to form a reflector standard wavefront. The interferometer furtherincludes a detector arranged so as to detect interference fringes causedby interference between the measurement wavefront and the referencewavefront. The detection of the interference fringes takes into accountthe reflector standard wavefront.

[0018] A second aspect of the invention is a method of manufacturing aprojection optical system capable of projecting a pattern from a reticleonto a photosensitive substrate. The method comprises the steps of firstmeasuring a shape of a test surface of an optical element that is acomponent of the projection optical system by causing interferencebetween light from the test surface and light from an aspheric referencesurface while the test surface and the aspheric reference surface areheld integrally and in close proximity to one another. The next step isassembling the optical element in the projection optical system andmeasuring the wavefront aberration of the projection optical system. Thenext step is then determining an amount by which the shape of the testsurface should be corrected based on the measured wavefront aberrationobtained in the step b. Then, the final step is correcting the shape ofthe test surface based on the amount by which the shape of the testsurface should be corrected as determined above.

[0019] A third aspect of the invention is an interferometer formeasuring wavefront aberration of an optical system having an objectplane and an image plane. The interferometer comprises a light sourcefor supplying light of a predetermined wavelength, a first pinholemember capable of forming a first spherical wavefront from the lightarranged at one of the object plane and the image plane. The firstpinhole member has a plurality of first pinholes arrayed in twodimensions along a surface perpendicular to an optical axis of theoptical system. The interferometer further includes a second pinholemember arranged at the opposite one of the object plane and the imageplane of the first pinhole member. The second pinhole member has aplurality of second pinholes arrayed at a position corresponding to theimaging position where the plurality of first pinholes is imaged by theoptical system. The interferometer also includes a diffraction gratingarranged in the optical path between the first and second pinholemembers, and a diffracted light plate member that selectively transmitsdiffracted light of one or more higher predetermined diffraction ordersassociated with the diffraction grating. The interferometer alsoincludes a detector arranged to detect interference fringes arising fromthe interference between a second spherical wavefront generated by azeroeth diffraction order passing through the second pinhole member andthe one or more higher predetermined diffraction orders passing throughthe diffracted light plate member.

[0020] A fourth aspect of the invention is an interferometer calibrationmethod for measuring a surface shape of an optical element of an opticalsystem. The method comprises the steps of first, interferometricallymeasuring the surface shape of the optical element to obtain a surfaceshape measurement value, then assembling the optical system by includingthe optical element in the optical system, then measuring a wavefrontaberration of the optical system, then separating the wavefrontaberration into a component corresponding to positional error of thesurface shape and a component corresponding to surface shape error, thencorrecting the positional error component and calculating the surfaceshape error component, and then finally correcting the surface shapemeasurement value using the surface shape error component as previouslycalculated

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is a schematic optical diagram of a first conventionalsurface-shape-measuring interferometer according to the prior art;

[0022]FIG. 2 is a schematic optical diagram of a second conventionalsurface-shape-measuring interferometer according to the prior art;

[0023]FIGS. 3a and 3 b are schematic optical diagrams of first andsecond surface-shape-measuring interferometers of a first embodimentaccording to a first aspect of the present invention;

[0024]FIGS. 4a and 4 b are schematic optical diagrams of third andfourth surface-shape-measuring interferometers of a first embodimentaccording to a first aspect of the present invention;

[0025]FIGS. 5a and 5 b are schematic optical diagrams of fifth and sixthsurface-shape-measuring interferometers of a second embodiment accordingto a first aspect of the present invention;

[0026]FIG. 6 is a schematic optical diagram of a seventhsurface-shape-measuring interferometer of a third embodiment accordingto a first aspect of the present invention;

[0027]FIG. 7 is a schematic optical diagram of an eighthsurface-shape-measuring interferometer of a fourth embodiment accordingto a second aspect of the present invention;

[0028]FIGS. 8a and 8 b are cross-sectional diagrams of the maincomponents of the holder assembly of the surface-shape-measuringinterferometer of FIG. 7;

[0029]FIG. 9 is a schematic optical diagram of a ninthsurface-shape-measuring interferometer that is a variation of thesurface-shape-measuring interferometer of FIG. 7;

[0030]FIG. 10a is a schematic optical diagram of a firstwavefront-aberration-measuring interferometer for explaining theprinciple of a fifth embodiment according to a third aspect of thepresent invention;

[0031]FIG. 10b is a cross-sectional diagram of a second semitransparentfilm with a pinhole plate in the interferometer of FIG. 10a;

[0032]FIG. 11a is a schematic optical diagram of a secondwavefront-aberration-measuring interferometer that is a variation of thewavefront-aberration-measuring interferometer of FIG. 10a;

[0033]FIG. 11b is a plan view of the second dual hole plate in theinterferometer of FIG. 11a;

[0034]FIG. 11c is a cross-sectional diagram explaining the operation ofthe second dual hole plate in the interferometer of FIGS. 11a and 11 b;

[0035]FIG. 12 is a schematic optical diagram of a thirdwavefront-aberration-measuring interferometer of a fifth embodimentaccording to a third aspect of the present invention;

[0036]FIG. 13a is a plan view of a first embodiment of the first pinholearray plate of the interferometer of FIG. 12;

[0037]FIG. 13b is a plan view of a first embodiment of the second dualhole array plate of the interferometer of FIG. 12;

[0038]FIG. 14a is a plan view of a second embodiment of the firstpinhole array plate, being a variation on the first embodiment of thefirst pinhole array plate of FIG. 13a;

[0039]FIG. 14b is a plan view of a second embodiment of the second dualhole array plate, being a variation on the first embodiment of thesecond dual hole array plate of FIG. 13b;

[0040]FIG. 15a is a schematic optical diagram of fourthwavefront-aberration-measuring apparatus of a sixth embodiment accordingto the present invention;

[0041]FIG. 15b is a plan view of second Hartmann plate of the apparatusshown in FIG. 15a;

[0042]FIG. 16a is a schematic optical diagram of a fifthwavefront-aberration-measuring interferometer of a seventh embodimentaccording to a third aspect of the present invention;

[0043]FIG. 16b is a plan view of the first pinhole cluster plate of thein interferometer of FIG. 16a;

[0044]FIG. 16c is a plan view of the second dual hole cluster plate ofthe in interferometer of FIG. 16a;

[0045]FIG. 17a is a plan view of the first pinhole row plate of aneighth embodiment according to a third aspect of the present invention;

[0046]FIG. 17b is a plan view of the second dual hole row plate in aneighth embodiment according to a third aspect of the present invention;

[0047]FIG. 18a is a plan view of the first slit plate of a ninthembodiment according to a third aspect of the present invention;

[0048]FIG. 18b is a plan view of the second dual slit plate of a ninthembodiment according to a third aspect of the present invention;

[0049]FIG. 19 is a schematic optical diagram of a sixthwavefront-aberration-measuring interferometer of a tenth embodimentaccording to a third aspect of the present invention;

[0050]FIG. 20a is a schematic optical diagram of seventhwavefront-aberration-measuring interferometer of an eleventh embodimentaccording to a third aspect of the present invention;

[0051]FIG. 20b is a cross-sectional diagram of the second pinhole mirrorplate in the interferometer of FIG. 20a;

[0052]FIG. 21a is a plan view of the first pinhole array plate used in avariation of the interferometer of FIG. 20a;

[0053]FIG. 21b is a plan view of second pinhole mirror array plate 63 ina variation on interferometer 22Q shown in FIG. 20a;

[0054]FIG. 22 is a schematic optical diagram of an eighthwavefront-aberration-measuring interferometer that is a variation of theinterferometer of FIG. 20a;

[0055]FIG. 23 is a schematic optical diagram of awavefront-aberration-measuring apparatus serving as a comparativeexample for illustrating the advantage of interferometers of FIGS. 20aand 22;

[0056]FIG. 24 is a flowchart indicating an exemplary method forcalibrating the aspheric-surface-shape measuring interferometer of FIG.7 using the wavefront-aberration-measuring interferometer FIG. 10a; and

[0057]FIG. 25 is a cross-sectional showing a small tool grindingapparatus used in the interferometer calibration method indicated in theflowchart of FIG. 24.

DETAILED DESCRIPTION OF THE INVENTION

[0058] The present invention relates to an interferometer system formeasuring the shape of an aspheric surface of an optical element in anoptical system and for measuring the wavefront aberration of such anoptical system, particularly in connection with manufacture of aprojection optical system suited to for use in an exposure apparatusemploying soft-X-ray (EUV) exposure light.

[0059] Referring to FIGS. 3a and 3 b, the principle of operation of aninterferometer system according to a first aspect of the presentinvention is now discussed.

[0060] Compared with prior art interferometer 122 shown in FIG. 1, firstand second interferometer systems 22A and 22B, shown in FIGS. 3a and 3b, respectively, according to the first aspect of the present inventionhave a reflective standard 40 with a separately and accuratelycalibrated spherical reflective surface 42 arranged in place of testsurface (aspheric surface) 138 of test object 136 (see FIG. 1).

[0061] Interferometer 22A shown in FIG. 3a further differs from priorart interferometer 122 of FIG. 1 in that a wavefront 45 incident nullelement 32 is a spherical wavefront from a Fizeau lens 44, and in that aFizeau surface 46 is used as the reference surface. Fizeau lens 42 neednot be limited to a convergent system as shown, but may also be adivergent system.

[0062] Interferometer 22B shown in FIG. 3b is an example wherein awavefront incident null element 32 is a plane wave 26, as in the case ofprior art interferometer 122 shown in FIG. 1. A flat Fizeau surface 30of a Fizeau lens 28 is used as the reference surface. Interferometer 22Bdiffers from prior art interferometer 122 of FIG. 1 in that the lightbeam converted by null element 32 is a convergent light beam, and inthat it permits measurement of concave surfaces as well as convexsurfaces. A method of calibrating null wavefront in this case is to usea concave reflective surface to calibrate the wavefront as it divergesafter having first converged and then to reverse calculate the shape ofthe null wavefront 34 at the position where it is actually used (heavyline in drawing) based on the calibrated wavefront shape. High-precisioncalibration is possible if a pinhole interferometer (i.e., a pointdiffraction interferometer, hereinafter referred to as a “PDI,”discussed further below) is used to calibrate the concave reflectivesurface.

[0063] If the amount of asphericity of surface 42 is small, then theentire surface can be measured all at once. However, in the case of anaspheric surface that unfortunately generates interference fringesexceeding the resolution of the interferometer CCD, data for the entiresurface can be obtained in the same manner by applying the so-calledwavefront synthesis technique. This technique involves axiallydisplacing reflective standard 40 relative to null wavefront 34,conducting interferometric measurements on a plurality of annularwavefront data, and joining the redundant regions of each of the data sothey overlap without excess.

[0064] First Embodiment

[0065] Referring now to FIGS. 4a and 4 b, third and fourthsurface-shape-measuring interferometers 22C and 22D of a firstembodiment according to a first aspect of the present invention are nowdescribed, wherein a PDI 52 employing an ideal spherical wavefront froma point light source 54 is used to measure null element 32 in Fizeau(aspheric-surface-measuring) interferometer (i.e., first interferometersystem) 22A shown in FIG. 3a.

[0066] Interferometer 22C shown in FIG. 4a employs a divergent nullelement 32, and interferometer 22D shown in FIG. 4b employs a convergentnull element 32. The latter is adopted when calibrating the wavefront 34for measurement of a convex surface.

[0067] Since spherical wavefront 45 incident null element 32 ininterferometers 22C and 22D of FIGS. 4a and 4 b is an ideal sphericalwavefront from a point light source 54, it is possible to simultaneouslyascertain the shape of null wavefront 34 as well as the transmissioncharacteristics of null element 32.

[0068] Second Embodiment

[0069] Referring now to FIGS. 5a and 5 b, fifth and sixthsurface-shape-measuring interferometers 22E and 22F of a secondembodiment according to a first aspect of the present invention are usedto measure null element 32 generating a convergent null wavefront 34, asthe case at interferometer 22B shown in FIG. 3b. Interferometer 22E ofFIG. 5a uses a spherical wavefront 45 as the wavefront from Fizeausurface 46 incident null element 32. Interferometer 22F in FIG. 5b usesa plane wave 26 therefor. It does not matter whether spherical wavefront45 is a convergent light beam or a divergent light beam. Furthermore,use of PDI 52 replaces calibration using a reflective surface. PDI 52corresponds to a point light source of the present invention.

[0070] To perform measurements with PDI 52, taking the case in whichnull wavefront 34 is convergent, pinhole 54 of PDI 52 is positioned soas to approximately coincide with the point of convergence of nullwavefront 34. As a result, null wavefront 34, which is reflected from areflective surface 56 surrounding pinhole 54, and the ideal sphericalwavefront produced by the light leaving pinhole 54 will forminterference fringes.

[0071] Third Embodiment

[0072] Referring now to FIG. 6. a seventh surface-shape-measuringinterferometer 22G is a third embodiment according to a first aspect ofthe present invention and is similar to interferometer 22E of FIG. 5a,except that a PDIs 52A is used in place of a Fizeau lens 44 that therehad generated a spherical wavefront. A second PDI 52B is also used formeasurement light. In interferometer 22E and 22F shown in FIGS. 5a and 5b, respectively, there is a possibility that during operation of Fizeauinterferometer 22E or 22F, the measurement light signal from PDI 52 willbe lost in noise. In this case, it is preferable to in addition employ apolarizing element to reduce noise and improve the usable signal.

[0073] The measurement arrangement in interferometer 22G shown in FIG. 6has the advantage that pinhole 54B that forms the point light source ofsecond PDI 52B acts to reduce noise and improve the usable signal. Thispermits not only the shape of null wavefront 34 and the transmissioncharacteristics of null element 32 to be accurately calibrated, but alsopermits the transmission characteristics of two PDIs 52A and 52B to becalibrated in both the forward and backward directions. Accordingly,accuracy can be further improved.

[0074] To actually use one of the aforementioned interferometers 22C-22Gto measure a test surface 38 after calibration has thus been performed,reflective standard 40, point light source forming means, PDIs 52 or thelike are removed and these are replaced with the original test surface38 and a light source 48, following which measurements may be performed.

[0075] As described above, interferometers 22C-22G of the first throughthird embodiments according to a first aspect of the present inventionmake it possible to calibrate an aspheric null element 32 with highprecision and in a short period of time.

[0076] Fourth Embodiment

[0077]FIG. 7 shows an eighth surface-shape-measuring interferometer 22Hof a fourth embodiment according to a second aspect of the presentinvention. FIGS. 8a and 8 b show the principal parts of interferometer22H of FIG. 7. Interferometer 22H shown in FIG. 7 is capable ofmeasuring the shape of an aspheric surface.

[0078] Referring to FIG. 7, laser light from a laser 24 is changed intoa collimated beam of a prescribed diameter by way of a lens system 58.and is then incident null element 32. Null element 32 emits a wavefronthaving a shape substantially identical to that of test surface 38, andthe wavefront, having been converted to a prescribed aspheric surfaceshape, is incident in perpendicular fashion, an aspheric referencesurface 70 and aspheric test surface 38. Furthermore, aspheric referencesurface 70 has substantially the same shape as aspheric test surface 38(with, however, concavity and convexity reversed). The light incidentaspheric reference surface 70 is amplitude-divided, with one wavefrontproceeding to test surface 38 and the other wavefront returning alongthe original optical path to serve as reference wavefront.

[0079] Aspheric reference surface 70 is arranged proximate test surface38, and aspheric reference surface 70 and test surface 38 have mutuallycomplementary shapes. Aspheric reference surface 70 and test surface 38are supported in integral fashion by a holder 72.

[0080] Furthermore, light from aspheric reference surface 70 isreflected by test surface 38, and is again incident aspheric referencesurface 70 as the measurement wavefront.

[0081] After the abovementioned reference wavefront and measurementwavefront exit from the reference optical element 76 upon which asphericreference surface 70 is formed, they are incident null element 32, arereflected by a beam splitter 74 within lens system 58, and then forminterference fringes on detector 60 comprising a CCD or other such imagepickup element. By processing these interference fringes with a computerCU electronically connected to detector 60, the shape error of testsurface 38 can be measured.

[0082] In interferometer 22H shown in FIG. 7, a main body, whichincludes the elements from laser 24 to null element 32, and holder 72,are supported by separate members so as to be spatially separated.

[0083] Interferometer 22H shown in FIG. 7 is basically a Fizeauinterferometer, but it has several significant advantages over prior artFizeau interferometer 222 of FIG. 2. The causes of the degradation inthe measurement reproducibility in a conventional interferometer such asinterferometer 122 of FIG. 1 or interferometer 222 of FIG. 2 include airfluctuations, vibration, sound, air pressure fluctuations, temperaturefluctuations, detector noise, nonlinear errors and amplitude errors inthe fringe scan, reproducibility of positioning the specimen,reproducibility of strain in the specimen due to the specimen holder,and aberrations in the optical system. Among these, air fluctuations,vibration, sound, air pressure fluctuations, temperature fluctuations,and optical system aberrations can be significantly reduced by bringingtest surface 38 and reference surface 70 close together and physicallyjoining them in integral fashion, as in interferometer 22H of the fourthembodiment of the present invention shown in FIG. 7.

[0084] Particularly with respect to interferometer 22H of in FIG. 7,while null element 32 is used therein, measurement accuracy is notaffected by either the accuracy of null element 32 or the accuracy ofalignment between null element 32 and test surface 38. This is becausenull element 32 functions to deliver a wavefront having an asphericshape more or less identical to aspheric reference surface 70 to thataspheric reference surface 70, but does not directly function to deliveran aspherically shaped wavefront to test surface 38. Accordingly,although null element 32 is not an essential component in interferometer22H, it is preferable to use null element 32 so as to improvemeasurement accuracy.

[0085] The positional reproducibility of test object 36 ininterferometer 22H is ensured through use of a position sensor PS(electronic micrometer or the like), not shown, arranged around testobject 36, and the reproducibility of strain in the test specimen 36from the specimen holder 72 is improved by constructing the specimenholder 72 such that support is effected in three-point or multi-pointfashion.

[0086] In addition, the close proximity of test surface 38 and referencesurface 70 makes detection of alignment error easier and enableshigh-precision alignment. Detector noise can be sufficiently reduced bycooling detector 60 and by integrating the data. Nonlinear errors andamplitude errors during fringe scans can be eliminated by using adigital-readout piezoelectric element, and by processing the signal suchthat there are an increased number of packets during fringe scans.Adoption of the above-described constitution in interferometer 22Hpermits attainment of repeatabilities of 0.05 nm RMS or better, andpermits attainment of measurement reproducibilities, including alignmenterror, changes occurring over time, and so forth, of 0.1 nm RMS orbetter.

[0087] A remaining problem with interferometer 22H is absolute accuracy,which is dependant on the surface accuracy of reference aspheric surface70. This error is a systematic error associated with the interferometer22H. Below are described ways to correct this error (i.e., howcalibration to offset this error.

[0088] Interferometer 22H, while based on conventional Fizeauinterferometer 222 shown in FIG. 2, is different from the conventionalFizeau interferometer in the following respects. Fizeau (reference)surface 70 of interferometer 22H is an aspheric surface, its shape beingsuch that convexity and concavity are reversed with respect to testsurface 38 arranged in close proximity to Fizeau surface 70. Theconstitution is such that reference element 76 is separated from theoptical system, and such that the (Fizeau) reference optical element 76is physically connected in integral fashion to test object 36. Thisconstitution significantly improves repeatability and measurementreproducibility as compared with that of above-described conventionalinterferometer 222 shown in FIG. 2.

[0089]FIGS. 8a and 8 b show two exemplary configurations for holderassembly 72 of interferometer 22H of FIG. 7. FIG. 8a shows an exemplaryconfiguration wherein the spacing between test surface 38 and asphericreference surface 70 is variable, and FIG. 8b shows an exemplaryconfiguration wherein the spacing is fixed.

[0090] Referring to FIG. 8a, reference element 76 with asphericreference surface 70 is held by reference element holder 72H, which isdisposed separately from the interferometer 22H main body. Apiezoelectric element 72P is provided on reference element holder 72H. Atest object holder 72T, which holds test object 36, is mounted toreference element holder 72H by way of piezoelectric element 72P. Bydriving piezoelectric element 72P, the spacing between asphericreference surface 70 and test surface 38 can be adjusted. Furthermore,this variable spacing can also be exploited to perform a fringe scan,which is a conventional method of analyzing interference fringes.

[0091] The exemplary configuration of holder assembly 72 shown in FIG.8b is similar to the exemplary configuration shown in FIG. 8a in thatreference element 76 with aspheric reference surface 70 is held byreference element holder 72H. However, holder assembly 72 of FIG. 8b hasspacers 72S directly vacuum-deposited at three locations on asphericreference surface 70. Spacers 72S are 1 to 3 μm in thickness, thisthickness being identical at all three locations. Furthermore, spacers72S are provided so that they trisect the circumference about an axis Axin the vertical direction of the paper surface in FIG. 8b. Test surface38 is mounted on (three) spacers 72S. The spacing between asphericreference surface 70 and test surface 38 can thereby be kept constantand the strain in test surface 38 due to gravity can also be keptconstant. If the exemplary configuration shown in FIG. 8b is employed,it is possible to perform a fringe scan for analyzing interferencefringes by varying laser wavelength, which has the additional benefit ofeliminating the likelihood that the interferometer will be affected bymechanical vibration or the like.

[0092] It is preferable that test object 36 be held in holder assembly72 in the same manner as it is held in the optical system of which it isan optical component. It is also preferable that test object 36 be heldin holder assembly 72 in the same orientation with respect to gravity asit is held in the optical system of which it is an optical component.This will make it possible to carry out meaningful measurements despitechanges in surface shape which may occur due to the action of strain ontest surface 38 when test object 36 is actually incorporated into anoptical system.

[0093] It is also preferable to make the spacing between asphericreference surface 70 and test surface 38 less than 1 mm. If this spacingexceeds 1 mm, the impact of air fluctuations, vibration, sound, airpressure fluctuations, temperature fluctuations and optical systemaberrations increases, leading to a deterioration in measurementaccuracy. To further improve measurement accuracy, it is preferable toset the spacing between aspheric reference surface 70 and test surface38 to be less than 100 μm.

[0094] In addition, if the spacing between aspheric reference surface 70and test surface 38 is fixed as in FIG. 8b, it is preferable to set thisspacing to be less than 1 μm.

[0095] Variation on Fourth Embodiment

[0096] In the exemplary configuration shown in FIG. 8a and discussedabove, the spacing between test surface 38 and aspheric referencesurface 70 may be detected by the following techniques.

[0097] Referring now also to FIG. 9, a ninth surface-shape-measuringinterferometer 22I is a variation on the above-described interferometer22H of the fourth embodiment shown in FIG. 7. In interferometer 22I,elements similar in function to elements as those in interferometer 22Hhave been given the same reference numerals and so a description thereofis omitted.

[0098] Interferometer 22I shown in FIG. 9 differs from interferometer22H shown in FIG. 7 in that a shearing interferometer 80 is providedbehind test surface 38 (i.e., at the side opposite from asphericreference surface 70). Shearing interferometer 80 guides light from awhite light source 80S to test surface 38 and aspheric reference surface70 by way of a beam splitter 80BS. Light reflected by test surface 38and light reflected by aspheric reference surface 70 passes through beamsplitter 80BS, and is horizontally displaced by a birefringent member80BR. The latter may be, for example, a Wollaston prism. The light thenpasses through an analyzer 80A and forms an interference pattern ondetector 60, such as a CCD. The spacing between test surface 38 andaspheric reference surface 70 can be detected by monitoring the changein the interference pattern on detector 60. In addition, ininterferometer 22I shown in FIG. 9, optical element 36 having testsurface 38 is preferably made of an optically transmissive material suchas, for example, quartz or Zerodur.

[0099] Fifth Embodiment

[0100] Referring now to FIGS. 10a-14, we describe a fifth embodimentaccording to a third aspect of the present invention. FIGS. 10a and 11 ashow first and second wavefront-aberration-measuring interferometers 22Jand 22K. FIGS. 12-14 b show exemplary configurations of a thirdwavefront-aberration-measuring interferometer 22L according to the fifthembodiment.

[0101] Interferometers 22J, 22K, and 22L, respectively shown in FIGS.10a, 11 a, and 12, are not “Fizeau” aspheric-surface-shape-measuringinterferometers for measuring the surface shape of a test surface 38 ofa test object 36 previously removed from an optical system of which itis an optical component, as were interferometers 122, 222, and 22A-22Ishown in FIGS. 1-9. Rather, they are wavefront-aberration-measuringinterferometers for measuring the wavefront aberration produced by anoptical system. Note that for the sake of convenience, the term“interferometer” is used to refer to either anaspheric-surface-shape-measuring interferometer, awavefront-aberration-measuring interferometer, or to both, when themeaning is clear from context.

[0102] The wavefront-aberration-measuring interferometers 22J-22Laccording to the fifth embodiment of the present invention use lightcorresponding to an exposure wavelength in the soft X-ray region tomeasure wavefront aberration of a projection optical system.

[0103] Referring to FIGS. 10a-11 c, the principle of thewavefront-aberration-measuring interferometer of the fifth embodimentaccording to a second aspect of the present invention is now described.

[0104] With reference to FIG. 10a, light from a synchrotron orbitalradiation (hereinafter “SOR”) undulator (not shown) passes through aspectroscope (not shown) to form quasimonochromatic light 84 having awavelength around 13 nm. Light 84 is condensed by a condenser mirror 64and is incident a first pinhole plate 86. First pinhole plate 86 has anaperture (pinhole) 86 o of a size smaller than the size of the Airy diskas determined from the numerical aperture on the incident side (firstpinhole plate 86 side) of an optical system 37 under test. The size ofthe Airy disk is given by 0.6 λ/NA, where NA is the incident-sidenumerical aperture of optical system 37, and λ is the wavelength ofquasimonochromatic light 84.

[0105] Light having a wavefront which can be regarded as that of anideal spherical wavefront will exit first pinhole plate 86. Light fromfirst pinhole plate 86 is then incident optical system 37, and thenarrives at a pinhole plate 88 having an aperture 88 o arranged at animage plane IP of optical system 37. First pinhole plate 86 and secondpinhole plate 88 are arranged at locations made mutually conjugate byoptical system 37, i.e., at locations corresponding to what would be anobject point and an image point if optical system 37 were actually usedto image an object.

[0106] Referring to FIG. 10b, pinhole plate 88 comprises asemitransparent film 88F provided on a substrate 88S which is opticallytransmissive at the wavelength of emitted quasimonochromatic light 84,and aperture 88 o wherein semitransparent film 88F is not provided.Accordingly, a portion of the wavefront incident pinhole plate 88 istransmitted without alteration of the wavefront, and another portionundergoes diffraction at aperture 88 o. Accordingly, if the size ofaperture 88 o is sufficiently small, the light diffracted at aperture 88o can be regarded as an ideal spherical wavefront.

[0107] Referring again to FIG. 10a, detector 60 is arranged on the exitside of pinhole plate 88 (i.e., at the side thereof opposite fromoptical system 37). Interference fringes are formed on detector 60 dueto interference between the ideal spherical wavefront from aperture 88 oand the transmitted wavefront from semitransparent film 88F. Thetransmitted wavefront from semitransparent film 88F corresponds in shapeto the wavefront aberration of optical system 37. The interferencefringes on detector 60 assume a shape corresponding to the deviation ofthis transmitted wavefront from an ideal spherical wavefront (ie., thewavefront from aperture 88 o). Accordingly, the wavefront aberration ofoptical system 37 can be determined by analyzing, in a computer CUelectrically connected to detector 60, the interference fringes detectedby detector 60.

[0108]FIG. 11a is a fourth wavefront-aberration-measuring interferometer22K employing an SOR undulator light source and which is a variation ofwavefront-aberration-measuring interferometer 22J of FIG. 10a. Note thatin FIGS. 11a-11 c, elements similar in function to elements appearing inFIGS. 10a and 10 b are given the same reference numerals as in FIGS. 10aand 10 b. Interferometer 22K makes use of a measurement technique ofhigher precision than that of interferometer 22J. Interferometer 22K inFIG. 11a differs from interferometer 22J in FIG. 10a in that a seconddual hole plate 90 is arranged in place of second pinhole plate 88, anda diffraction grating 62 is inserted between first pinhole plate 86 andsecond dual hole plate 90.

[0109]FIG. 11b shows the constitution of second dual hole plate 90, andFIG. 11c is a diagram for explaining the functions of diffractiongrating 62 and second dual hole plate 90. Referring to FIG. 11b, seconddual hole plate 90 has microscopic aperture 90 o that functions as apinhole, and an aperture 92 that is larger than pinhole 90 o. Pinhole 90o and aperture 92 are formed such that if second dual hole plate 90 isat the location of image plane IP of optical system 37, pinhole 90 o ispositioned in the optical path of the zeroeth-order peak P0 of thediffraction pattern produced by diffraction grating 62. In addition,aperture 92 is positioned in the optical path of a first-order peak P1of the diffraction pattern produced by diffraction grating 62, as shownin FIG. 11c.

[0110] Accordingly, zeroeth-order peak P0 is diffracted by pinhole 90 o,forming an ideal spherical wavefront 45 which then proceeds to detector60. In addition, a wavefront 45′ associated with first-order peak P1,which contains information about the wavefront aberration of opticalsystem 37, passes through aperture 92 without alteration, and proceedsto detector 60. At this time, zeroeth-order peak P0 and first-order peakP1 have wavefronts 45 and 45′, respectively, corresponding to thewavefront aberration of optical system 37. Wavefront 45 of the lightthat passes through pinhole 90 o, is converted to an ideal sphericalwavefront. However, wavefront 45′ passing through aperture 92 does notundergo any significant amount of diffraction, and so has a wavefrontshape corresponding to the wavefront aberration of optical system 37.Accordingly, interference fringes due to interference between idealspherical wavefront 45 from pinhole 90 o and measurement wavefront 45from aperture 92 are formed on detector 60. The profile of theinterference fringes formed on detector 60 will correspond to thedeviation of the measurement wave from an ideal spherical wavefront 45,and wavefront 45′ containing aberration information of optical system 37can be determined by analyzing these interference fringes, as in thecase for interferometer 22J of FIG. 10a.

[0111] With continuing reference to FIG. 11a, a fringe scan forhigh-precision measurement can be performed by moving diffractiongrating 62 by operatively connecting the latter to a diffraction gratingdriving unit DU. In interferometer 22K, diffraction grating 62 is shownarranged in the optical path between optical system 37 and second dualhole plate 90. However, diffraction grating 62 may be arranged in theoptical path anywhere between first pinhole plate 86 and second dualhole plate 90. For example, it is possible to arrange diffractiongrating 62 in the optical path between first pinhole plate 86 andoptical system 37. In addition, while the above-described embodiment ofinterferometer 22K employed two diffraction orders P0 and P1 of thediffraction pattern produced by diffraction grating 62. the presentinvention is not limited to two such orders or of combinations of thezeroeth-order and first-order.

[0112] Referring now to FIG. 12, a fifth wavefront-aberration-measuringinterferometer 22L, which represents a fifth embodiment according to thesecond aspect of the present invention for measuring the wavefrontaberration of an optical system 37 based on the principle explainedabove with reference to FIGS. 10a-11 c, is now described. In FIG. 12,elements similar in function to elements appearing in FIGS. 10a-11 c aregiven the same reference numerals as in FIGS. 10a and 10 b.

[0113] In interferometers 22J and 22K shown in FIGS. 10a and 11 a, theaberration of optical system 37 can only be measured at one point inimage plane IP. To accurately ascertain the aberration of an opticalsystem, it is necessary to measure a plurality of image points. Tomeasure a plurality of image points in interferometers 22J and 22K, oneconceivable method of performing measurements would involve moving firstpinhole plate 86 and second pinhole plate 88. or second dual hole plate90, to a number of prescribed positions. In this case, since thepinholes are extremely small, there is a risk that the pinholes will beaffected by the vibration of the movement mechanism that moves thepinholes, and that particularly for pinholes on the image side, it willnot be possible to make light pass through these pinholes stably. Thismakes good measurements extremely problematic. In addition, if pinholesare moved, it becomes difficult to measure the pinhole positions withgood accuracy. Further, there is a risk that the accuracy with whichaberration (particularly distortion), is measured will no longer besufficient, particularly for image points.

[0114] In interferometer 22L, a first pinhole array plate 93, whereinpinholes are arrayed in two dimensions, is used in place of firstpinhole plate 86 of interferometer 22K shown in FIG. 11a.

[0115] Referring to FIG. 12, light from an SOR undulator (not shown)passes through an analyzer (not shown) to form quasimonochromatic light84 having of wavelength around 13 nm. This light is condensed bycondenser mirror 64 and is incident first pinhole array plate 93. Unlikewavefront-aberration-measuring interferometers 22J and 22K shown inFIGS. 10a and 11 a, interferometer 22L shown in FIG. 12 is constitutedsuch that light is incident the image plane IP side, not the objectplane OP side, of optical system 37, the reason for which is discussedbelow.

[0116] Turning briefly to FIG. 13a, first pinhole array plate 93comprises an array or matrix of pinhole apertures (pinholes) 93 o of asize well smaller than the size of the Airy disk 0.6 λ/NA, as determinedfrom the numerical aperture (imagewise numerical aperture) NA at theincident side of optical system 37. The positions of pinholes 93 ocorrespond to the locations of image points of optical system 37 forwhich measurement of wavefront aberration is desired.

[0117] Returning now to FIG. 12, condenser mirror 64 is provided on acondenser mirror stage 66, which is capable of movement parallel toimage plane IP of optical system 37. By moving condenser mirror stage66, any desired pinhole 93 o on first pinhole array plate 93 can beselectively illuminated. An illuminated pinhole 93 o corresponds to ameasurement point. Furthermore, the position at which quasimonochromaticlight 84 is incident first pinhole array plate 93 will change with themovement of condenser mirror stage 66. In addition, it is also possibleto collectively illuminate a plurality of pinholes 93 o on first pinholearray plate 93 instead of, or in addition to, illuminating just one ofthe pinholes. Nonetheless, in the description below, it is assumed forthe sake of convenience, that only one pinhole 93 o is illuminated.

[0118] Referring now also to FIG. 13b, second dual hole array plate 94is located in object plane OP, ie., arranged at the position at whichoptical system 37 images first pinhole array plate 93. Second dual holearray plate 94 has a plurality of pinhole apertures (pinholes) 94 oprovided in a matrix at positions at which the plurality of pinholes 93o of first pinhole array plate 93 are imaged, and a plurality ofapertures 95 provided in a matrix such that each is separated by aprescribed distance from each of the plurality of pinholes 94 o.Furthermore, each of the plurality of pinholes 94 o has the samefunction as pinholes 90 o in FIG. 11b, and each of the plurality ofapertures 95 has the same function as aperture 92 in FIG. 11b.

[0119] Referring again to FIG. 12, light having a wavefront 45, whichcan be regarded as that of an ideal spherical wavefront, exits anilluminated pinhole 93 o, and is incident optical system 37. This lightpasses through optical system 37 and is diffracted by diffractiongrating 62 arranged between optical system 37 and object plane IP.Zeroeth-order peak P0 (not shown in FIG. 12) of the diffraction patternarrives at pinhole 94 o on second dual hole array plate 94 correspondingto the illuminated pinhole 93 o on first pinhole array plate 93.First-order peak P1 (not shown on FIG. 12) of the diffraction patternarrives at aperture 94 o on second dual hole array plate 94corresponding to the illuminated pinhole 93 o on first pinhole arrayplate 93. Light that passes through pinhole 94 o and the light thatpasses through aperture 95 mutually interfere.

[0120] With continuing reference to FIG. 12, detector 60, is attached toa detector stage 68 which is capable of movement parallel to objectplane OP, is arranged at the exit side of second dual hole array plate94. Detector stage 68 is constituted so that it is linked with and moveswith condenser mirror stage 66, and such that only pinhole 94 o andapertures 95, corresponding to illuminated pinhole 93 o, can be seenfrom detector 60. Accordingly, the interference fringes due to the lightonly from pinhole 94 o and aperture 95, corresponding to the illuminatedpinhole 93 o, are formed on detector 60. By analyzing these interferencefringes, the wavefront aberration at image plane IP locationcorresponding to illuminated pinhole 93 o can be determined.

[0121] In interferometer 22L of FIG. 12, first pinhole array plate 93and second dual hole array plate 94 are physically grounded (i.e.,secured so as to be stationary) with respect to optical system 37. Thus,stable measurements can be performed without being affected byvibrations caused by the movement of stages 66, 68 during actualmeasurements.

[0122] First pinhole array plate 93 is mounted on a vertical stage 67,which is capable of causing first pinhole array plate 93 to move injogged (i.e., incremental) fashion in a direction parallel to theoptical axis of optical system 37. Vertical stage 67 is preferablysecured to the same frame (not shown) that supports optical system 37.In addition, second dual hole array plate 94 is mounted on an XY stage69, which is capable of causing second dual hole array plate 94 to movein jogged fashion within object plane OP of optical system 37. XY stage69 is attached to the abovementioned frame by way of a piezoelectricelement. Adjustment of focus can be performed by using vertical stage 67to move first pinhole array plate 93. If there is distortion in opticalsystem 37, XY stage 69 can be used to align the position of pinhole 94o. Furthermore, a length measuring interferometer or other suchmicrodisplacement sensor (not shown) may be provided on XY stage 69,permitting distortion in optical system 37 to be measured based on theoutput from the microdisplacement sensor. Furthermore, in the presentembodiment, the positions of the plurality of pinholes 93 o of firstpinhole array plate 93 and the plurality of pinholes 94 o of second dualhole array plate 94 are accurately measured beforehand using acoordinate measuring interferometer.

[0123] Although the position of pinhole 94 o is moved in interferometer22L, this pinhole can be positioned with good accuracy since the strokeof this movement is small. Furthermore, interferometer 22L isconstituted such that pinhole 94 o, on the object plane OP side ofoptical system 37 is moved when optical system 37 has a reductionmagnification of −1/β. Thus, the positioning accuracy of pinhole 94 ocan be relaxed by the factor |−1/β| as compared with the case in whichpinhole 93 o, on the image plane IP side of optical system 37, is moved.

[0124] Interferometer 22L is not constituted so that pinhole 93 o ismoved and the amount of movement of pinhole 94 o is in a range whereinpositioning accuracy can be maintained. Thus, stable measurement can beachieved, and the measurement accuracy of aberration, particularlydistortion, at the imaged location can be made sufficient.

[0125] In interferometer 22L shown in FIG. 12, the plurality of pinholes93 o corresponding to positions for measurement of the wavefrontaberration of optical system 37 are shown arranged in a matrix. However,the arrangement of pinholes 93 o is not limited to a typical square orrectangular matrix. For example, referring to FIG. 14a, if the field(exposure field) EF of optical system 37 is arcuate, as shown in FIGS.14a and 14 b, then a pinhole plate 93′ having pinholes 93 o may bearranged with a prescribed spacing at an object height (image height) ofthe same height as that of optical system 37. Also, as shown in FIG.14b, the arrangement of the pinholes 94 o and apertures 95 in seconddual hole array plate 94′ will have to be prealigned with pinholes 93 oof the first pinhole array plate 93.

[0126] While diffraction grating 62 in interferometer 22L of FIG. 12 isarranged in the optical path between optical system 37 and second dualhole array plate 94, diffraction grating 62 may also be arranged in theoptical path between first pinhole array plate 93 and second dual holearray plate 94. For example, it is possible for diffraction grating 62to be arranged between first pinhole array plate 93 and optical system37. In addition, while interferometer 22L shown in FIG. 12 employs twopeaks of the diffraction pattern produced by diffraction grating 62,i.e., zeroeth-order peak P0 of the diffraction pattern and first-orderpeak P1 of the diffraction pattern, the present invention is not limitedto employment of two such peaks, or of employment of combinations ofzeroeth-order and first-order peaks.

[0127] Sixth Embodiment

[0128] Referring now to FIGS. 15a and 15 b, a fourthwavefront-aberration-measuring apparatus 22M of a sixth embodimentaccording to the present invention is now described. Apparatus 22M usesa soft X-ray exposure wavelength to measure the wavefront aberration ofan optical system 37. Note that in FIGS. 15a and 15 b, elements similarin function to elements appearing in FIGS. 10a-14 b are given the samereference numerals as in FIGS. 10a-14 b.

[0129] Referring to FIG. 15a, light from an SOR undulator (not shown)passes through an analyzer (not shown) to form quasimonochromatic light84 having a wavelength around 13 nm, which is condensed by a condensermirror 64 and is incident first pinhole plate 86. First pinhole plate 86has an aperture of a size well smaller than the size of the Airy disk,0.6 λ/NA, where λ is the wavelength of quasimonochromatic light 84 andNA is the numerical aperture on the incident side (first pinhole plate86 side) of optical system 37. Accordingly, the light that exits firstpinhole plate 86 can be regarded as having the wavefront of an idealspherical wavefront.

[0130] In apparatus 22M, a second Hartmann plate 96 having a pluralityof apertures 96 o, as shown in FIG. 15b, is arranged between thelocation of image plane IP of optical system 37 (a location madeconjugate to first pinhole plate 86 by optical system 37) and opticalsystem 37.

[0131] Returning to FIG. 15a, the light beam from first pinhole plate86, upon exiting optical system 37, forms, due to the action of theplurality of apertures 96 o of second Hartmann plate 96, a plurality ofray groups RG that are the same in number as the number of apertures 96o. Ray groups RG then proceed to image plane IP of optical system 37.Ray groups RG converge at image plane IP of optical system under test 37and arrive at detector 60 in a divergent state. If the plane of thepupil (not shown) of optical system under test 37 is subdivided into aplurality of sections, ray groups RG that pass through the plurality ofapertures 96 o on second Hartmann plate 96 respectively correspond torays passing through each such pupil section. As a result, the lateralaberration of optical system 37 can be determined if the position atwhich each of ray groups RG arrives at detector 60 is detected. Thewavefront aberration of optical system 37 can be determined from thislateral aberration.

[0132] In apparatus 22M, the plurality of apertures 96 o provided onsecond Hartmann plate 96 are arranged in a matrix as shown in FIG. 15b.However, the present invention is not limited to this arrangement. Inaddition, while in apparatus 22M second Hartmann plate 96 is arrangedbetween optical system 37 and image plane IP second Hartmann plate 96may also be located between first pinhole plate 86 and image plane IP,it being possible, for example, for second Hartmann plate 96 to bearranged in the optical path between first pinhole plate 86 and opticalsystem 37.

[0133] Seventh Embodiment

[0134] Referring now to FIGS. 16a-16 c, a fifthwavefront-aberration-measuring interferometer 22N of a seventhembodiment according to a third aspect of the present invention isdescribed. Interferometers 22J, 22K, 22L, and 22M of the fifth and sixthembodiments discussed above are wavefront-aberration-measuringinterferometers which employ an SOR undulator (not shown) as a lightsource. Although accuracy can be made extremely high if an SOR undulatoris used as a light source, the apparatus itself becomes excessivelylarge, and it is generally extremely difficult to use in a factory.Thus, referring to FIG. 16a, in interferometer 22N discussed in furtherdetail below, a laser plasma X-ray (hereinafter “LPX”) source 21 is usedin place of an SOR undulator as light source. LPX source 21 generateshigh-temperature plasma from a target 25 when high-intensity pulsedlaser light is focused on target 25. X-rays present within this plasmaare then used. In interferometer 22N, light emitted from LPX source 21is divided into spectral components by a spectroscope (not shown), andlight 27 of only a prescribed wavelength (e.g., 13 nm) is extracted.Light 27 is used as the light for wavefront-aberration-measuringinterferometer 22N.

[0135] The intensity of LPX source 21 is smaller than that of the SORundulator by an order of magnitude. Consequently, in interferometer 22N,first pinhole plate 86, which had only a single aperture ininterferometers 22J, 22K, 22L, and 22M of the fifth and sixthembodiments shown in FIGS. 10a-15 b and discussed above, is replacedwith a first pinhole cluster plate 87. The latter includes a pluralityof pinhole clusters 87 c, each of which contains a plurality of pinholes87 o clustered together in a microlocation, as shown in FIG. 16b.

[0136] Referring again to FIG. 16a, in LPX source 21, a laser lightsource 23 supplies high-intensity pulsed laser light of a wavelength inthe range from the infrared region to the visible region. Laser lightsource 23 may be, for example, a YAG laser excited by a semiconductorlaser, an excimer laser, or the like. This laser light is condensed by acondenser optical system 29 onto target 25. Target 25 receives thehigh-intensity laser light, rises in temperature and is excited to theplasma state, and emits X-rays 27 during transitions to a lowerpotential state. By passing X-rays 27 through a spectroscope (notshown), quasimonochromatic light 27 only of wavelength 13 nm isextracted, which is then acted on by condenser mirror 64 and irradiatesa pinhole cluster 87 c on first pinhole cluster plate 87.

[0137] Referring again to FIG. 16b, first pinhole cluster plate 87 haspinhole clusters 87 c, each of which comprises a plurality of pinholes87 o clustered in a microlocation at a position for which the wavefrontaberration of optical system 37 is to be measured. Note that in FIG.16b, pinhole cluster 87 c is shown as having only four pinholes 87 o.However, pinhole cluster 87 c preferably actually comprises one hundredor more pinholes 87 o. Pinholes 87 o are of a size much smaller than thesize of the Airy disk 0.6 λ/NA, where λ is the wavelength ofquasimonochromatic light 27 and NA is the numerical aperture on theincident side (first pinhole cluster plate 87 side) of optical system37. In addition, FIG. 16b shows an exemplary schematic arrangementwherein a plurality of pinhole clusters 87 c are formed on first pinholecluster plate 87. In practice the positions at which pinhole clusters 87c are formed to correspond to the positions of object points of opticalsystem 37 for which measurement is desired.

[0138] Returning to FIG. 16a, the entire region of one pinhole cluster87 c on first pinhole cluster plate 87 is illuminated byquasimonochromatic light 27. A plurality of ideal spherical wavefrontsare generated from the numerous pinholes 87 o of the illuminated pinholecluster 87 c. The plurality of ideal spherical wavefronts passes throughoptical system 37, and then proceeds to and converges at image plane IPof optical system 37, which position is made conjugate to first pinholecluster plate 87 by optical system 37.

[0139] Although not shown in FIGS. 16a-16 c, in interferometer 22N oneof pinhole clusters 87 c on first pinhole cluster plate 87 isselectively illuminated, just as in the case of interferometers 22J,22K, and 22L of the fifth embodiment, discussed above.

[0140] In interferometer 22N diffraction grating 62 is arranged betweenoptical system 37 and the location of the image plane IP of opticalsystem 37. The light that exits optical system 37 and passes throughdiffraction grating 62 is diffracted by diffraction grating 62 andproceeds to a second dual hole cluster plate 89.

[0141]FIG. 16c shows a preferred constitution of second dual holecluster plate 89. Second dual hole cluster plate 89 has pinhole cluster89 c comprising a plurality of pinholes 89 o provided in one-to-onecorrespondence with the pinholes 87 o of which plurality of pinholeclusters 87 c on first pinhole cluster plate 87 are each comprised, anda plurality of apertures 89 a provided in one-to-one correspondence withthe plurality of pinhole clusters 87 c. In other words, one aperture 89a corresponds to one pinhole cluster 87 c comprising a plurality ofpinholes 87 o.

[0142] At this time, if second dual hole cluster plate 89 is arranged atimage plane IP, then plurality of pinhole clusters 89 c and plurality ofapertures 89 a will be positionally related so that pinhole cluster 89 cis positioned in the optical path of the zeroeth-order peak P0 of thediffraction pattern produced by diffraction grating 62, and so thataperture 89 a is positioned in the optical path of first-order peak P1of the diffraction pattern produced by diffraction grating 62.

[0143] Accordingly, the ideal spherical wavefronts from pinhole cluster87 c on first pinhole cluster plate 87 pass through optical system 37and are then diffracted by diffraction grating 62. Of the light producedby this diffraction, zeroeth-order peak P0 of the diffraction patternarrives at the pinhole cluster 89 c on second dual hole cluster plate89, which corresponds to illuminated pinhole cluster 87 c. In addition,first-order peak P1 of the diffraction pattern arrives at the aperture89 a on second dual hole cluster plate 89, which corresponds toilluminated pinhole cluster 87 c. Zeroeth-order peak P0 of thediffraction pattern and first-order peak P1 of the diffraction patternhave wavefronts corresponding in shape to the wavefront aberration ofoptical system 37. Zeroeth-order peak P0 of the diffraction pattern isdiffracted by pinhole cluster 89 c as it passes therethrough and isconverted to a second group of ideal spherical wavefronts. First-orderpeak P1 of the diffraction pattern passes through aperture 89 a andexits therefrom without being diffracted. The light from the secondideal spherical wavefront group and the light from aperture 89 amutually interfere.

[0144] Accordingly, interference fringes due to interference between theideal spherical wavefront group from pinhole cluster 89 c and thewavefront from aperture 89 a are formed on detector 60 arranged on theexit side of second dual hole cluster plate 89 (i.e., on the side ofsecond dual hole cluster plate 89 opposite from optical system 37).Furthermore, the interference fringes on detector 60 form a shapecorresponding to the deviation from an ideal spherical wavefront of thewavefront that passes through optical system 37. The wavefrontaberration of optical system 37 can be determined by analyzing theseinterference fringes via computer CU electrically connected to detector60, just as in the previously mentioned embodiments.

[0145] Furthermore, although not shown in FIG. 16a, detector 60 isconstituted so as to be capable of movement parallel to image plane IPof optical system 37 so that it can be made to selectively receive thelight from pinhole cluster 89 c and aperture 89 a corresponding toilluminated pinhole cluster 87 c, just as in interferometers 22J, 22K,and 22L of the fifth embodiment, discussed above. As a result, wavefrontaberration can be measured at a plurality of measurement points withinobject plane OP of optical system 37.

[0146] The seventh embodiment of the present invention as describedabove can provide a wavefront-aberration-measuring interferometer 22Nthat can be used even in an ordinary factory.

[0147] Furthermore, while diffraction grating 62 in interferometer 22Nof the seventh embodiment shown in FIG. 16a is arranged in the opticalpath between optical system 37 and second dual hole cluster plate 89,diffraction grating 62 may also be arranged in the optical path betweenfirst pinhole cluster plate 87 and second dual hole cluster plate 89. Itbeing possible, for example, to arrange diffraction grating 62 in theoptical path between first pinhole cluster plate 87 and optical system37. Also, while interferometer 22N employs two peaks of the diffractionpattern produced by diffraction grating 62 (zeroeth-order peak P0 andfirst-order peak P1) the present invention is not limited to employmentof two such peaks or of employment of combinations of the zeroeth-orderand first-order peaks.

[0148] Eighth Embodiment

[0149] Referring now to FIGS. 17a and 17 b, an eighth embodimentaccording to a third aspect of the present invention is described.Interferometer 22N of the seventh embodiment shown in FIG. 16a anddescribed above employed pinhole clusters 87 c, 89 c provided with aplurality of pinholes 87 o, 89 o in prescribed microlocations. However,a pinhole row plate 97 may be used, wherein plate 97 includes aplurality of a pinhole rows 97R wherein a plurality of pinholes 97 o arearranged unidimensionally in a prescribed direction, as shown in FIG.17a. In this case, first pinhole row plate 97 is provided with aplurality of rows 97R of pinholes 97 o arrayed in matrix-like fashion soas to correspond to a plurality of measurement points in object plane OPor image plane IP of optical system 37. Although FIG. 17a shows apinhole row 97R having only four pinholes 97 o. an actual pinhole row97R comprises 100 or more pinholes 97 o. Pinholes 97 o are of a sizesmaller than the Airy disk 0.6 λ/NA, where λ is the wavelength ofquasimonochromatic light 84 and NA is the numerical aperture on theincident side of optical system 37 (i.e., on the side thereof at whichfirst pinhole row plate 97, which here takes the place of first pinholecluster plate 87 shown in FIG. 16a, is present).

[0150] Referring back and forth between FIGS. 16a-16 c and FIGS. 17a-17b, if first pinhole row plate 97 is used in place of first pinholecluster plate 87, then a second dual hole row plate 99 should be used inplace of second dual hole cluster plate 89. Second dual hole row plate99 has a plurality of pinhole rows 99R, each of which comprises aplurality of pinholes 99 o provided in one-to-one correspondence withpinholes 97 o of which pinhole rows 97R on first pinhole row plate 97are each comprised. In addition, plate 99 has a plurality of apertures99 a provided in one-to-one correspondence with plurality of pinholerows 97 o. Furthermore, each of the plurality of pinhole rows 99Rcomprises numerous pinholes 99 o arrayed unidimensionally in aprescribed direction. In addition, one aperture 99 a corresponds to onepinhole row 97R comprising plurality of pinholes 97 o.

[0151] Employment of a pinhole row 97R, 99R thus comprising a pluralityof pinholes 97 o, 99 o arrayed unidimensionally in a prescribeddirection makes it possible to reduce noise caused by the intermixing oflight among the plurality of pinholes 92 o, 94 o, 93 o, 95 o, 96 o, 87o, 89 o, and measurement accuracy can thereby be further improved.

[0152] It is also preferable to make the pitch of the plurality ofpinholes arrayed unidimensionally in a prescribed direction be 10 to 25times the radius of the Airy disk 0.6 λ/NA as determined by thenumerical aperture on the first pinhole row plate 97 side of opticalsystem 37. It is further preferable to make it approximately 16 to 20times this Airy disk radius.

[0153] Ninth Embodiment

[0154] Referring now to FIGS. 18a and 18 b, we describe a ninthembodiment according to a third aspect of the present invention. It ispossible to use slit-shaped apertures 57 s, 59 s in place of pinholeclusters 87 c, 89 c in interferometer 22N shown in FIG. 16a anddescribed above. FIGS. 18a and 18 b show slit plates 57, 59 providedwith pluralities of slit-shaped apertures 57 s, 59 s.

[0155] In describing the use of first slit plate 57 and second dual slitplate 59 in place of first pinhole cluster plate 87 and second dual holecluster plate 89, to reference is made back and forth between FIGS.16a-16 c and FIGS. 18a-18 b.

[0156] In FIG. 18a, first slit plate 57 is provided with a plurality ofslit-shaped apertures 57 s arrayed in matrix-like fashion so as tocorrespond to a plurality of measurement points in object plane OP imageplane IP of optical system 37. Furthermore, the slit shape mentioned inthe present embodiment refers to a shape extending unidimensionally in aprescribed direction, the overall shape hereof not being limited torectangular. In addition, the width in the latitudinal direction ofslit-shaped aperture 57 s is of a size well smaller than the size of theAiry disk 0.6 λ/NA, where λ is the wavelength of quasimonochromaticlight 27 and NA is the by numerical aperture on the incident side (onthe side of first slit plate 57, which here corresponds to first pinholecluster plate 87 in FIG. 16a) of optical system 37. Upon illumination ofa slit-shaped aperture 57 s, the wavefront emitted therefrom will besuch that its cross section in the short direction of the slit-shapedaperture 57 s is the same as that of an ideal spherical wavefront (i.e.,this then can be said to represent a one-dimensional ideal sphericalwavefront).

[0157] If first slit plate 57 shown in FIG. 18a is used in place offirst pinhole cluster plate 87 shown in FIG. 16b, then second dual slitplate 59 shown in FIG. 18b should be used in place of second dual holecluster plate 89. Second dual slit plate 59 comprises a plurality ofslit-shaped apertures 59 s provided in one-to-one correspondence withthe plurality of slit-shaped apertures 57 s on first slit plate 57, anda plurality of apertures 59 a provided in one-to-one correspondence withthe plurality of slit-shaped apertures 57 s on first slit plate 57.

[0158] In the ninth embodiment of the invention, slit plates 57, 59shown in FIGS. 18a and 18 b are incorporated inwavefront-aberration-measuring interferometer 22N of the seventhembodiment shown in FIG. 16a. Operation in this case is as follows.

[0159] First, one of the plurality of slit-shaped apertures 57 s firstslit plate 57 corresponding to a desired measurement point isilluminated with light 27 from LPX source 21. The wave emitted from theilluminated slit-shaped aperture 57 s is such that a one-dimensionalideal spherical wavefront is generated in the short direction ofslit-shaped aperture 57 s. This one-dimensional ideal sphericalwavefront passes through optical system 37 and is diffracted bydiffraction grating 62. Zeroeth-order peak P0 of the diffraction patternarrives at the corresponding slit-shaped aperture 59 s on second dualslit plate 59, and first-order peak P1 of the diffraction patternarrives at aperture 59 a on second dual slit plate 59.

[0160] Furthermore, a one-dimensional ideal spherical wavefront isgenerated in the short direction of the corresponding slit-shapedaperture 59 s on second dual slit plate 59, and a wavefrontcorresponding in shape to the wavefront aberration of optical system 37passes through aperture 59 a. The wavefront of the one-dimensional idealspherical wavefront and the wavefront from the aperture 59 a mutuallyinterfere and form interference fringes on detector 60. The wavefrontaberration of optical system 37 can be measured by analyzing theseinterference fringes in computer CU. Furthermore, it is possible in thisninth embodiment that measurement accuracy will lower in a directionparallel to the long direction of slits 57 s, 59 s. If this should bethe case, all that need be done to rectify this is to arrange slitplates 57, 59 and optical system 37 such that they are rotatablerelative to one another, or to provide a plurality of slit-shapedapertures 57 s, 59 s having long directions in mutually differentorientations in place of the slit-shaped apertures 57 s, 59 s shown inFIGS. 18a and 18 b.

[0161] Thus, by using slit-shaped apertures 57 s, 59 s, it is possibleto further increase light flux as compared with cases wherein pinholeplates having a single pinhole, or a pinhole cluster or a pinhole rowcomprising a plurality of pinholes, are used. This constitutioncorresponds to a shearing interferometer.

[0162] Also, while second dual slit plate 59 makes use of two peaks ofthe diffraction pattern produced by diffraction grating 62(zeroeth-order peak P0 and first-order peak P1), the present inventionis not limited to employment of two such peaks or of employment ofcombinations of the zeroeth-order and first-order peaks thereof.

[0163] Tenth Embodiment

[0164] Referring to FIG. 19, we describe a sixthwavefront-aberration-measuring interferometer 22P of a tenth embodimentaccording to a third aspect of the present invention.

[0165] Interferometer 22P is a variation on the above-discussedinterferometers 22M, 22N in the sixth embodiment shown in FIGS. 15a-16c. An LPX source 21 is used in interferometer 22P of the tenthembodiment in place of the SOR undulator light source (not shown) thatwas used in interferometers 22M, 22N of the sixth embodiment.

[0166] Referring to FIG. 19, in LPX source 21, laser light source 23supplies pulsed laser light of a wavelength in the range from theinfrared region to the visible light region. Laser light source 23 maybe, for example, a YAG laser excited by a semiconductor laser, anexcimer laser, or the like. This laser light is condensed by condenseroptical system 29 onto target 25. Target 25 receives the high-intensitylaser light, rises in temperature and is excited to the plasma state,and emits X-rays 27 during transitions to a lower potential state. Bypassing X-rays 27 through a spectroscope (not shown), quasimonochromaticlight 27 only of wavelength 13 nm is extracted, which is then acted onby condenser mirror 64 and irradiates a pinhole plate 31.

[0167] Pinhole plate 31 has a single aperture much larger (i.e., ten ormore times) than the diameter of the Airy disk 0.6 λ/NA, where λ is thewavelength of quasimonochromatic light 27 and NA is the numericalaperture on the incident side (pinhole plate 31 side) of optical system37. Here, so long as aperture 31 o of pinhole plate 31 can beilluminated such that there is uniform illuminance within object planeOP of optical system 37 and such that there is uniform illuminancewithin the cross section of the light beam incident pinhole plate 31,there is no need to make the size of the aperture of pinhole plate 31smaller than the Airy disk, as is the case for the above-describedembodiments.

[0168] In interferometer 22P, illumination is such that there is uniformilluminance within object plane OP and within the cross section of thelight beam incident pinhole plate 31. Accordingly, the pinhole plate 31which is used can have a large aperture 31 o such as has been described.

[0169] As in the case in the above-described embodiments, ininterferometer 22P, light exiting from aperture 31 o of pinhole plate 31can be regarded as having an ideal spherical wavefront.

[0170] As in the case in interferometer 22M, in interferometer 22P,second Hartmann plate 96 (see FIG. 15b) having a plurality of apertures96 o is arranged between image plane IP of optical system 37 (i.e., alocation made conjugate to pinhole plate 31 by optical system 37) andoptical system 37.

[0171] With continuing reference to FIG. 19, the light beam fromaperture 31 o of pinhole plate 31, upon exiting from optical system 37,forms, due to the action of the plurality of apertures 96 o of secondHartmann plate 96, a plurality of ray groups RG that are the same innumber as the number of apertures 96 o. Ray groups RG then proceed toimage plane IP of optical system 37. Ray groups RG converge at imageplane IP and arrive at detector 60 in a divergent state. If the plane ofthe pupil (not shown) of optical system 37 is subdivided into aplurality of sections, ray groups RG that pass through the plurality ofapertures 96 o on second Hartmann plate 96 respectively correspond torays passing through each such section. As a result, the lateralaberration of optical system 37 can be determined if the position atwhich each of the ray groups RG arrives at detector 60 is detected. Thewavefront aberration of optical system 37 can then be determined fromthis lateral aberration using computer CU, as describe above.

[0172] Eleventh Embodiment

[0173] Referring now to FIGS. 20a and 20 b, a seventhwavefront-aberration-measuring interferometer 22Q in an eleventhembodiment according to a third aspect of the present invention isdescribed.

[0174] Although a light source 21 supplying light in the soft X-raywavelength region was used as light source in the above-describedinterferometers 22N-22P in the seventh through tenth embodiments, it maybe convenient to use an ordinary laser light source 41 (see FIG. 20a),not an X-ray source 21 (see FIGS. 16a and 19). when assembling andadjusting optical system 37 at an ordinary factory.

[0175]FIG. 20a shows wavefront-aberration-measuring interferometer 22Qof the tenth embodiment which uses a non-X-ray laser light source 41.FIGS. 20a-23 are intended to assist in explaining the principle of theeleventh embodiment.

[0176] Referring to FIG. 20a, in interferometer 22Q, laser light source41 supplies laser light of a prescribed wavelength. This laser light issplit by a beam splitter 74 adjacent light source 41. One of the beamsb1 so split travels by way of two folding mirrors 35 a and 35 b to acondenser lens 39, and is guided to first pinhole plate 86 having asingle pinhole 86 o. First pinhole plate 86 is arranged at the locationof image plane IP of optical system 37. Pinhole 86 o is of a sizesmaller than the diameter of the Airy disk 0.6 λ/NA, where λ is thewavelength of the laser light and NA is the numerical aperture NA on theincident side (first pinhole plate 86 side) of optical system 37.Accordingly, a first ideal spherical wavefront is generated from pinhole86 o of first pinhole plate 86.

[0177] The first ideal spherical wavefront from first pinhole plate 86passes through optical system 37 and is guided to second pinhole mirrorplate 33 arranged at a position conjugate to first pinhole plate 86 byoptical system 37.

[0178] Referring to FIG. 20b second pinhole mirror plate 33 comprises anoptically transparent substrate 33S, reflective surface 33R provided onsubstrate 33S, and aperture 33 o, which is a region wherein reflectivesurface 33R is not provided. Furthermore, aperture 33 o of secondpinhole mirror plate 33 is of a size smaller than the diameter of theAiry disk 0.6 λ/NA, where λ is the wavelength of the laser light and NAis the numerical aperture on the exit side (second pinhole mirror plate33 side) of optical system 37.

[0179] Returning again to FIG. 20a, light beam b2 produced by splittingat beam splitter 74 travels by way of a folding mirror 35 c to passthrough a condenser lens 49, and is then guided in a condensed state tothe rear side of second pinhole mirror plate 33R (i.e., the backthereof, if the side on which reflective surface 33R is applied is takenas the front thereof), which is arranged in object plane OP of opticalsystem 37.

[0180] Accordingly, a second ideal spherical wavefront will be generatedat second pinhole mirror plate 33 when light beam b2 from the rear sideof second pinhole mirror plate 33 passes through aperture 33 o. Inaddition, the light beam that passes through optical system 37 isreflected by reflective surface 33R of second pinhole mirror plate 33.This reflected light has a wavefront corresponding in shape to thewavefront aberration of optical system 37.

[0181] The second ideal spherical wavefront from aperture 33 o of secondpinhole mirror plate 33 and the reflected light from reflective surface33R of second pinhole mirror plate 33 arrive at detector 60 by way oflens 47, and form interference fringes on detector 60.

[0182] The interference fringes on detector 60 form a shapecorresponding to the deviation from an ideal spherical wavefront of thewavefront that passes through optical system 37. The wavefrontaberration of optical system 37 can be determined by analyzing theseinterference fringes using computer CU, as described above.

[0183] In FIGS. 20a and 20 b, which illustrate the principle of thewavefront-aberration-measuring interferometer 22Q of the eleventhembodiment, one prescribed point in object plane OP (or image plane IP)of optical system 37 is used as the measurement point. If a plurality ofmeasurement points are to be measured, then, referring briefly to FIG.21a. first pinhole array plate 61 wherein a plurality of pinholes 61 oare arranged in a prescribed array may be used in place of first pinholeplate 86 of FIG. 20a. In addition, a second pinhole mirror array plate63 having a plurality of pinholes 63 o and a reflective interstitialsurface 63R may be used in place of second pinhole mirror plate 33 shownin FIGS. 16a and 16 b.

[0184] Referring now to FIG. 22, an eighthwavefront-aberration-measuring interferometer 22R, which is a variationon wavefront-aberration-measuring interferometer 22Q of the eleventhembodiment wherein the wavefront aberration of optical system 37 can bemeasured at a plurality of measurement points, is described. In FIG. 22,elements similar in function to elements appearing in FIG. 20a have beengiven the same reference numerals as in FIG. 20a and description thereofwill be omitted here for the sake of convenience.

[0185] Referring to FIG. 22 and interferometer 22R, laser light of aprescribed wavelength from laser light source 41 is split by beamsplitter 74. One of the light beams b1 so split sequentially travels byway of folding mirror 35 a to condenser lens 39 provided on condenserlens stage 66 capable of movement parallel to the image plane of opticalsystem 37, thereafter arriving at first pinhole array plate 61.

[0186] Referring back to FIG. 21a, first pinhole array plate 61 has aplurality of pinholes 61 o arrayed in a matrix. The positions of theplurality of pinholes 61 o correspond to the positions of measurementpoints for optical system 37. Furthermore, each of the plurality ofpinholes 61 o is of a size smaller than the diameter of the Airy disk0.6 λ/NA, where λ is the wavelength of the laser light and the NA is thenumerical aperture on the incident side (first pinhole array plate 61side) of optical system 37. Accordingly, upon being illuminated, pinhole61 o on first pinhole array plate 61 will generate an ideal sphericalwavefront.

[0187] Returning again to FIG. 22, as a result of moving condenser lensstage 66, a desired pinhole 61 o on first pinhole array plate 61 isselectively illuminated. Furthermore, the position at which the laserlight is incident folding mirror 35 a mounted on condenser lens stage 66changes as condenser lens stage 66 is moved. In addition, instead of oneof pinholes 61 o, a plurality of pinholes 61 o may also be collectivelyilluminated.

[0188] With continuing reference to FIG. 22, the ideal sphericalwavefront from first pinhole array plate 61 passes through opticalsystem 37, and is then guided to second pinhole mirror array plate 63,located at a position conjugate to first pinhole array plate 61 byoptical system 37.

[0189] Referring briefly again to FIG. 21b, second pinhole mirror arrayplate 63 is provided with reflective interstitial surface 63R arrangedsuch that plurality of pinholes 63 o form a matrix, no such reflectiveinterstitial surface 63R being provided at the locations of pinholes 63o. Furthermore, each of the plurality of pinholes 63 o of second pinholemirror array plate 63 is of a size smaller than the diameter of the Airydisk 0.6 λ/NA, where λ is the wavelength of the laser light and NA isthe numerical aperture on the exit side (second pinhole mirror arrayplate 63 side) of optical system 37.

[0190] Returning now to FIG. 22, light beam b2 produced by splitting atbeam splitter 74 sequentially travels by way of oscillatory foldingmirror 45 electrically connected to mirror oscillating unit MU, and thenby way of folding mirror 35 to a condenser lens 49, and is then guidedin a condensed state to the rear side of second pinhole mirror arrayplate 63 (i.e., the side opposite from the side at which reflectiveinterstitial surface 63R is present), which is arranged in object planeOP of optical system 37.

[0191] Accordingly, an ideal spherical wavefront is generated at secondpinhole mirror array plate 63 when light beam b2 from the rear side ofsecond pinhole mirror array plate 63 passes through pinhole 63 o. Inaddition, when the light beam that passes through optical system 37 isreflected by reflective interstitial surface 63R of second pinholemirror array plate 63, the reflected light will have a wavefrontcorresponding in shape to the wavefront aberration of optical system 37.

[0192] The ideal spherical wavefront from pinhole 63 o of second pinholemirror array plate 63 and the light reflected by reflective interstitialsurface 63R of second pinhole mirror array plate 63 arrive at detector60 by way of another folding mirror 35 d and lens 47, and forminterference fringes on detector 60.

[0193] The interference fringes on detector 60 form a shapecorresponding to the deviation from an ideal spherical wavefront of thewavefront that passes through optical system 37. The wavefrontaberration of optical system 37 can be determined by analyzing theseinterference fringes using computer CU, as discussed above.

[0194] In interferometer 22R as a variation on the eleventh embodimentshown in FIG. 22, detector 60, along with the optical system whichguides the light from second pinhole mirror array plate 63 to detector60, and condenser lens 49 are mounted on Detector stage 68, which iscapable of movement parallel to object plane OP of optical system 37.Detector stage 68 is constituted so that it is linked and moves withcondenser lens stage 66 discussed above, and only pinhole 63 o,corresponding to the illuminated pinhole 61 o, can be seen from detector60.

[0195] Accordingly, interference fringes are formed on detector 60 dueto interference between the light that passes through optical system 37from illuminated pinhole 61 o and the diffracted light from pinhole 63 oon second pinhole mirror array plate 63 corresponding to the illuminatedpinhole 61 o. Accordingly, the wavefront aberration at the measurementpoint where the illuminated pinhole 61 o is positioned can be determinedby analyzing these interference fringes.

[0196] Stable measurement can also be performed with interferometer 22Rin this variation on the eleventh embodiment shown in FIG. 22, withoutbeing affected by vibrations caused by the movement of stages 66, 68during measurement.

[0197] With continuing reference to FIG. 22, first pinhole array plate61 is mounted on a vertical stage 67, which is capable of causing firstpinhole array plate 61 to move in jogged (i.e., incremental) fashion ina direction parallel to the optical axis of optical system 37. Verticalstage 67 is secured to the same frame that supports optical system 37.In addition, second pinhole mirror array plate 63 is mounted on an XYstage 69, which is capable of causing second pinhole mirror array plate63 to move in jogged fashion within object plane OP of optical system37. XY stage 69 is attached to the abovementioned frame by way of apiezoelectric element. Furthermore, adjustment of focus can be performedby using vertical stage 67 to move first pinhole array plate 61. Ifthere is distortion in optical system 37, XY stage 69 can be used toalign the position of pinhole 63 o.

[0198] Furthermore, a length measuring interferometer or other suchmicrodisplacement sensor is preferably provided on XY stage 69,permitting distortion in optical system 37 to be measured based on theoutput from the microdisplacement sensor. Furthermore, in the presentembodiment, the positions of the plurality of pinholes 61 o of firstpinhole array plate 61 and the plurality of pinholes 63 o of secondpinhole mirror array plate 63 are accurately measured beforehand using acoordinate measuring interferometer.

[0199] In addition, oscillatory folding mirror 45 in interferometer 22Rin this variation on the eleventh embodiment shown in FIG. 22 isconstituted so as to permit oscillation via mirror oscillation unit MU,the difference in lengths of the optical paths of the two beams producedby beam splitter 74 changing in accordance with this oscillation. As aresult, a fringe scan can be executed for high-precision measurement.

COMPARATIVE EXAMPLE

[0200] Referring to FIG. 23, wavefront-aberration-measuringinterferometer 22S is a comparative example for illustrating theadvantage of interferometers 22Q and 22R of the eleventh embodiment.Interferometer 22S of the comparative example shown in FIG. 23 employsan ultraviolet laser 41 instead of the SOR undulator light sourceemployed in interferometer 22J shown in FIG. 10a. As previouslymentioned, measurement accuracy increases as the wavelength of the lightsource of the wavefront-aberration-measuring interferometer isshortened. Since the wavelength of an ultraviolet laser 41 isapproximately 20 times longer than the working wavelength of opticalsystem 37, the accuracy of interferometer 22S of the comparative examplecan be expected to be 20 times worse than that of interferometer 22Jshown in FIG. 10a.

[0201] However, in interferometers 22Q and 22R of FIGS. 20a and A22, thereference wavefront is made to travel along an optical path separatefrom the measurement wavefront. Thus, measurement can be performed witha precision higher than is possible with interferometer 22S of thecomparative example shown in FIG. 23. Thus, in interferometers 22Q and22R of the eleventh embodiment, wavefront aberration can be measuredwith high precision without the need to use an X-ray source.

[0202] Method of Calibrating Aspheric-Shape-Measuring Interferometer

[0203]FIG. 24 is a flowchart for assisting in describing a method forcalibrating an aspheric-surface-shape measuring interferometer of thetype shown in FIGS. 1-7. In the course of this calibration, awavefront-aberration-measuring interferometer of the type shown in FIGS.10a-22 is used to verify the aspheric shape obtained using theaspheric-surface-shape measuring interferometer. This method orvariations thereof can be applied to any of these interferometers forthe sake of convenience, however, we take the example of calibration ofaspheric-surface-shape measuring interferometer 22H of the fourthembodiment shown in FIG. 7 using wavefront-aberration-measuringinterferometer 22J of the fifth embodiment shown in FIG. 10a.

[0204] Before executing step S1 in FIG. 24, the aspheric surface undertest 38 is first machined to a surface accuracy of approximately 10 nmRMS using well-known technology.

[0205] At step S1 in FIG. 24, the surface shape of the abovementionedaspheric test surface 38 is measured using interferometer 22H of thefourth embodiment of the present invention shown in FIG. 7. Furthermore,interferometer 22H of the fourth embodiment may also be used startingfrom the time when the aspheric surface is first machined. Whenperforming measurements using interferometer 22H, it is preferable tominimize asymmetric systematic errors (errors in reference surface 70)by collecting data at stepped angular rotations obtained by eitherrotating test surface 38 about the optical axis with respect toreference surface 70 in stepwise fashion or rotating reference surface70 about the optical axis with respect to test surface 38 in stepwisefashion, and averaging the data obtained.

[0206] At step S2, using the measurement data from step S1, correctivegrinding is performed on the aspheric surface 38 so as to make the shapeof aspheric test surface 38 conform to the design data. FIG. 25 shows asmall tool grinding apparatus 400 for performing this correctivegrinding. Referring to FIG. 25, small tool grinding apparatus 400 hasgrinding head 406 provided with a polisher 410 that rotates, and coilspring 414 that applies a prescribed pressure to polisher 414. Aspherictest surface 38 is ground as a result of application of a constant loadin a direction normal to aspheric test surface 38 as optical testelement 36 is rotated. The amount of grinding is proportional to thedwell time of polisher 410 (i.e., the time that polisher 410 remains ata given position and grinds). Furthermore, the shape of test surface 38is measured using interferometer 22H shown in FIG. 7, just as wasperformed at step S1. If the result of measurement is that the measuredaspheric shape differs from the design shape, the shape of test surface38 is again corrected using small tool grinding apparatus 82. Byrepeating this measurement and correction process, the measured asphericshape and the design aspheric shape can be made to coincide.

[0207] At step S3, optical element 36 having test surface 38 shaped as aresult of the operations at step S2 is assembled in the optical system37 of which it is an optical component.

[0208] At step S4, the wavefront aberration of the optical system 37assembled in step S3 is measured. In connection with the measurement ofthis wavefront aberration, a PDI (point diffraction interferometer)employing an SOR (synchrotron orbital radiation) undulator light source,such as in interferometer 22J shown in FIG. 10a, is used. Since themeasurement wavelength of interferometer 22J is short, at about 13 nm,the wavefront aberration of the optical system can be measured with highprecision, specifically to 0.13 nm RMS or better. The constitutions ofexemplary interferometers which may be applied here are described underthe fifth through eleventh embodiments of the present invention shown inFIGS. 10a-22.

[0209] At step S5, the causes of error in the wavefront aberrationmeasured at step S4 is broken down into an alignment error component(for each aspheric surface) and a shape error component for eachsurface.

[0210] Specifically, a computer uses, for example, known optical systemautomatic correction software, assigns the position of test surface 38(spacing, inclination and shift) and the shape of test surface 38 asvariables, initializes the measurement values of the wavefrontaberration, and performs optimization so that the wavefront aberrationapproaches zero. The difference between the position and shape of testsurface 38 when optimized and the position and shape of test surface 38prior to optimization corresponds to the alignment error (positionalerror) and shape error, respectively.

[0211] At step S6, the alignment error calculated at step S5 isevaluated to determine whether it is sufficiently small. If it is notsmall enough, the flow operation proceeds to step S7 where the alignmenterror is adjusted. If it is small enough, the flow proceeds to step S8.

[0212] At step S7, alignment of optical element 36 in optical system 37is adjusted based on the alignment error calculated at step S5,following which flow returns to step S4.

[0213] Note that the sequence of operations between steps S4 and S7 arerepeated until the alignment error calculated at step S5 is sufficientlysmall.

[0214] At step S8, the difference between the shape error (shape errorisolated by the most recent iteration of step S5) in the final wavefrontaberration (wavefront aberration as determined by the most recentiteration of step S4) and the final measured aspheric surface shape datacalculated in step S2 is calculated. This difference corresponds to thesystematic error of aspheric-surface-shape-measuring interferometer 22H.This error corresponds to the shape error of reference surface (Fizeausurface) 70 in the aspheric-surface-shape-measuring (Fizeau-type)interferometer 22H.

[0215] At step S9, the final aspheric surface shape data measured atstep S2 is corrected by the amount of the systematic error calculated atstep S8, and test surface 38 is reworked using small tool grindingapparatus 400 based on this corrected aspheric surface shape data. Atthis time, optical element 36 having test surface 38 is removed fromoptical system 37 of which it is a part before corrective grindingoperations can be carried out.

[0216] After steps S1 through S9 have been executed, optical system 37is reassembled and the wavefront aberration is measured usinginterferometer 22J shown in FIG. 10a. The measured values are againseparated into an alignment error component and a shape error componentfor each surface, and the surface error is verified to determine whetherit is smaller than previously measured.

[0217] By numerous repetitions of the series of procedures includingmachining of aspheric test surface 38, assembly in optical system 37,measuring of wavefront aberration, and determining the systematic errorin aspheric-surface-shape-measuring interferometer 22H as describedabove, systematic errors in aspheric-surface-shape-measuringinterferometer 22H can be identified. Furthermore, if such errors arelarge (e.g., 2 nm RMS or greater), aspheric-surface-shape-measuringinterferometer 22H must itself be corrected (i.e., the surface shape ofaspheric reference surface 70 must be corrected).

[0218] If the measurement values during subsequent measurements andmachining are continuously corrected by the amount of the systematicerror in aspheric-surface-shape-measuring interferometer 22H ascalculated by this procedure and this then used as data duringoperations using the corrective grinding apparatus 400, an asphericsurface 38 can be machined with good accuracy.

[0219] Since measurement accuracy, and in particular reproducibility,with aspheric-surface-shape-measuring interferometer 22H of the fourthembodiment are excellent, the above-described calibration method isextremely effective.

[0220] Furthermore, should existence of systematic errors be confirmedthereafter as a result of wavefront aberration measurement based at theexposure wavelength or other such measurements performed during aproduction run, systematic error may be corrected at each such occasionso as to constantly approach design values.

[0221] In addition, after machining the aspheric surface 38 using themachining and measurement procedures based on the present invention,optical system 37 is assembled and a reflective film (not shown) must beapplied to each surface 38 to be made reflective prior to measurement ofthe wavefront aberration. The shape of surface 38 may change under theinfluence of stress from the film when applying and removing (e.g., toperform corrective grinding) the reflective film. Although thereproducibility of this change should be less than 0.1 nm RMS, this isnot attainable. Nevertheless, the majority of the surface change issecond- and fourth-order components (power components and third-orderspherical aberration components). and the higher-order components aresmall. Second-order and fourth-order surface change components can becompensated to a certain degree by adjusting the surface spacing. Inother words, it is sufficient to ensure that the reproducibility of thesurface changes associated only with higher-order components are held to0.1 nm RMS or smaller. This can be accomplished by sufficient reductionof the stress from the film.

[0222] As described above, the present invention provides anaspheric-surface-shape measuring interferometer displaying goodreproducibility, and moreover makes it possible to measure wavefrontaberration with high precision. In addition, the present inventionpermits improvement in the absolute accuracy of precision surfacemeasurements in an aspheric-surface-shape measuring interferometer. Inaddition, the present invention permits manufacture of a projectionoptical system having excellent performance.

[0223] Adoption of the present invention also makes it possible toaccurately verify the shape of a null wavefront, as well as thetransmission characteristics of such a null wavefront, without the needto use a reflective standard. Moreover, adoption of an interferometersystem according to the present invention makes it possible to calibratean aspheric null element with high precision and in a short period oftime.

[0224] Furthermore, the wavefront-aberration-measuring interferometersof the fifth through eleventh embodiments of the present inventiondiscussed above can be assembled as part of an exposure apparatus. Inparticular, when an SOR undulator of a wavelength which may be used forexposure is used as light source in a wavefront-aberration-measuringinterferometers, as was the case in the fifth and sixth embodiments,this will be favorable since the light source unit can also serve as theexposure light source. When a laser plasma X-ray source of a wavelengthwhich may be used for exposure in a wavefront-aberration-measuringinterferometers, as was the case in the seventh through tenthembodiments, this will be favorable since the light source unit can alsoserve as the exposure light source. In addition, thewavefront-aberration-measuring interferometers of the eleventhembodiment of the present invention requires a laser light source to befurnished separate from the exposure light source. However, this laserlight source can also serve as light source for an alignment system oras light source for an autofocus system in the exposure apparatus. Inaddition, in the wavefront-aberration-measuring interferometers of thefifth through eleventh embodiments of the present invention, when thislight source is shared by the exposure apparatus, detector 60 serving asdetector may also be fashioned such that it is removable from theexposure apparatus. In this case, the wavefront aberration of projectionoptical system 37 can be measured by attaching such a removable unit tothe exposure apparatus in the event that maintenance or the like isrequired. Consequently, there will be no need to provide a dedicatedwavefront-aberration-measuring interferometer for each and everyexposure apparatus, permitting reduction in the cost of the exposureapparatus.

[0225] In addition, while detector 60 has been adopted as detector inthe fifth through tenth embodiments of the present invention discussedabove, a member having a function that converts emitted light in thesoft X-ray region to visible light (for example, a fluorescent plate)may be provided at the position of the detector 60 and used in placethereof, and the visible light from this member may be detected by adetector such as a CCD.

[0226] Furthermore, the embodiments of the present invention discussedabove describe a manufacturing method of a projection optical system 37in the context of an exposure apparatus that uses soft X-rays ofwavelength around 10 nm as exposure light,wavefront-aberration-measuring interferometers ideally suited to themeasurement of the wavefront aberration of this projection opticalsystem 37, surface-shape-measuring interferometers ideally suited tomeasurement of the surface shape of a reflective surface in thisprojection optical system 37, and a calibration method for such aninterferometer. However, the present invention is not limited to thissoft X-ray wavelength. For example, the present invention can be appliedto a projection optical system or wavefront-aberration-measuringinterferometer for hard X-rays of wavelength shorter than soft X-rays,and to a surface-shape-measuring interferometer that measures thesurface shape of an optical element of a hard X-ray projection opticalsystem, and can also be applied to the vacuum ultraviolet region (100 to200 nm) of wavelength longer than soft X-rays. Furthermore, measurementand manufacturing of a precision much greater than hitherto possiblebecomes possible if the present invention is applied to avacuum-ultraviolet projection optical system orwavefront-aberration-measuring interferometer, or to surface shapemeasurement of an optical element in a vacuum-ultraviolet projectionoptical system.

[0227] Thus, the present invention is not to be limited by the specificmodes for carrying out the invention described above. In particular,while the present invention has been described in terms of severalaspects, embodiments, modes, and so forth, the present invention is notlimited thereto. In fact, as will be apparent to one skilled in the art,the present invention can be applied in any number of combinations andvariations without departing from the spirit and scope of the inventionas set forth in the appended claims, and it is intended to cover allalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. An interferometer capable of measuring a surfaceshape of a target surface as compared to a reflector standard,comprising: a. a light source capable of generating a light beam; b. areference surface arranged downstream of said light source forreflecting said light beam so as to form a reference wavefront; c. anull element arranged downstream of said reference surface for forming adesired null wavefront from said light beam and arranged such that saidnull wavefront is incident the target surface so as to form ameasurement wavefront and is incident the reflector standard whenalternately arranged in place of the target surface so as to form areflector standard wavefront; and d. a detector arranged so as to detectinterference fringes caused by interference between said measurementwavefront and said reference wavefront taking into account saidreflector standard wavefront.
 2. An interferometer according to claim 1,wherein: a. the target surface is an aspheric surface; and b. saiddetector measures the shape of said measurement wavefront over a desiredrange by analyzing a plurality of interference fringe patternsrepresenting different subregions of said measurement wavefront whichare obtained by changing the positional relationship between saidmeasurement wavefront and said reflector standard.
 3. An interferometercapable of measuring a surface shape of a target surface as compared toa reflector standard, comprising: a. a point light source formed byirradiating a reflective surface so as to form an outgoing sphericalwave; b. a reference surface arranged downstream of said point lightsource for reflecting light from said light source so as to form areference wavefront; c. a null element arranged downstream of saidreference surface for forming a desired null wavefront and arranged suchthat said null wavefront is incident the target surface so as to form ameasurement wavefront, and the reflector standard alternately arrangedin place of the target surface so as to form a reflector standardwavefront; d. wherein said null wavefront is determined by reflectingand folding said measurement wavefront from said reflector standard bysaid reflecting surface, causing said measurement wavefront to interferewith said reference wavefront thereby causing interference; and e. adetector arranged so as to detect interference fringes caused by saidinterference.
 4. An interferometer according to claim 3, wherein: a. thetarget surface is an aspheric surface; and b. said detector measures theshape of said measurement wavefront over a desired range by analyzing aplurality of interference fringe patterns representing differentsubregions of said measurement wavefront which are obtained by changingthe positional relationship between said measurement wavefront and saidreflector standard.
 5. An interferometer capable of measuring a surfaceshape of a target surface as compared to a reflector standard,comprising: a. a point light source having a reflective surface, forforming an outgoing spherical wavefront, and that is alternatelyarrangeable in placed of the target surface; b. a reference surfacearranged downstream of said point light source for reflecting light fromsaid light source so as to form a reference wavefront; c. a null elementarranged downstream of said reference surface for forming a desired nullwavefront and arranged such that said null wavefront is incident thetarget surface so as to form a measurement wavefront, and the reflectorstandard alternately arranged in place of the target surface so as toform a reflector standard wavefront; d. wherein said null wavefront isdetermined by passing said spherical wavefront from said point lightsource through said null element and performing an interferometricmeasurement; and e. a detector arranged so as to detect interferencefringes created by the interference between said measured wavefront andsaid reference wavefront, taking into account said null wavefront.
 6. Aninterferometer capable of measuring a surface shape of a target surfaceas compared to a reflector standard, comprising: a. a point light sourceformed by irradiating a reflective surface so as to form an outgoingspherical wave; b. a reference surface arranged downstream of said pointlight source for reflecting light from said light source so as to form areference wavefront; c. a null element arranged downstream of saidreference surface for forming a desired null wavefront and arranged suchthat said null wavefront is incident the target surface so as to form ameasurement wavefront, and the reflector standard alternately arrangedin place of the target surface so as to form a reflector standardwavefront; d. wherein said null wavefront is determined by passing saidspherical wavefront from said point light source arranged in place ofthe target surface through said null element and performing aninterferometric measurement; and e. a detector arranged so as to detectinterference fringes created by the interference between said measuredwavefront and said reference wavefront, taking into account said nullwavefront.
 7. A method of manufacturing a projection optical systemcapable of projecting a pattern from a reticle onto a photosensitivesubstrate, comprising the steps of: a. measuring a shape of a testsurface of an optical element that is a component of the projectionoptical system by causing interference between light from said testsurface and light from an aspheric reference surface while said testsurface and said aspheric reference surface are held integrally and inclose proximity to one another; b. assembling said optical element inthe projection optical system and measuring the wavefront aberration ofthe projection optical system; c. determining an amount by which saidshape of said test surface should be corrected based on said measuredwavefront aberration obtained in said step b; and d. correcting saidshape of said test surface based on said amount by which said shape ofsaid test surface should be corrected as determined in said step c.
 8. Amethod according to claim 7, wherein: a. said step c further includesthe step of calculating an error in said shape of said test surface asmeasured in said step a, based on said measured wavefront aberrationobtained in said step b; and b. said amount by which said shape of saidtest surface should be corrected is determined from said calculatederror, said measured wavefront aberration and said shape of said testsurface.
 9. A method according to claim 7, wherein: a. said step ofcalculating an error in said shape of said test surface includesseparating said measured wavefront aberration into an test surfacepositional error component, a test surface shape error component, and aresidual component when said positional error component is substantiallycorrected; and b. wherein error in said surface shape is calculatedbased on a component in said residual component attributable to saidshape error component and said shape of said test surface.
 10. A Fizeauinterferometer for measuring the shape of an optical surface of anoptical element, comprising: a. a light source for forming a light beamalong an optical path; b. an aspherical reference surface arranged insaid optical path downstream from said light source; c. a null elementarranged in an said optical path; and d. a holding member that holdssaid reference surface and the optical surface as a single unit suchthat said reference surface and the optical surface are brought closetogether so that light from said reference surface and the opticalsurface interferes.
 11. An interferometer according to claim 10, furtherincluding a main body unit that supplies light to said reference surfaceand said optical surface, and that causes the interference of light thattravels via said reference surface and the light that travels via saidoptical surface, and wherein said holding member and said main body unitare spatially separated.
 12. An interferometer according to claim 10,wherein said reference surface and the optical surface are separated bya spacing that is less than 1 mm.
 13. An interferometer according toclaim 10, wherein said reference surface and the optical surface areseparated by a spacing that is variable.
 14. An interferometer accordingto claim 10, further including a position detection system that detectsthe positional relationship between said reference surface and theoptical surface.
 15. An interferometer according to claim 10, whereinsaid reference surface and the optical surface are separated by a fixedspacing that is less than 10 μm.
 16. An apparatus for measuringwavefront aberration of an optical system having an object plane and animage plane, comprising: a. a light source for supplying light of apredetermined wavelength; b. a first pinhole member capable of forming afirst spherical wavefront from said light arranged at one of said objectplane and said image plane, said first pinhole member having a pluralityof first pinholes arrayed in two dimensions along a surfaceperpendicular to an optical axis of the optical system; c. a secondpinhole member arranged at the opposite one of said object plane andsaid image plane of said first pinhole member, said second pinholemember having a plurality of second pinholes arrayed at a positioncorresponding to the imaging position where said plurality of firstpinholes is imaged by said optical system; d. a diffraction gratingarranged in the optical path between said first and second pinholemembers; e. a diffracted light plate member that selectively transmitsdiffracted light of one or more higher predetermined diffraction ordersassociated with said diffraction grating; and f. a detector arranged todetect interference fringes arising from the interference between asecond spherical wavefront generated by a zeroeth diffraction orderpassing through said second pinhole member and said one or more higherpredetermined diffraction orders passing through said diffracted lightplate member.
 17. An apparatus according to claim 16, wherein said lightsource is one among the group of light sources consisting of:synchrotron, laser and laser plasma X-ray.
 18. An apparatus according toclaim 17, wherein: a. said light source is a laser plasma X-ray source;and b. said first plurality of pinholes comprises pinhole groupscomprising a plurality of pinholes.
 19. An apparatus according to claim18, wherein said plurality of pinholes in said pinhole groups is arrayedalong a predetermined one-dimensional direction.
 20. An apparatusaccording to claim 19, wherein said plurality of pinholes has an arraypitch that is 10 to 25 times the Airy disk radius determined by thenumerical aperture on said first pinhole side of the optical system andsaid predetermined wavelength.
 21. An apparatus according to claim 18,wherein said plurality of pinholes constituting said pinhole group eachhas a slit-shaped aperture extending in a predetermined one-dimensionaldirection.
 22. An apparatus according to claim 16, further including: a.first selective illumination means for selectively illuminating a firstportion of said plurality of first pinholes of said first pinholemember; and b. selective light receiving means for selectively receivingsaid second spherical wavefront from a second portion of said pluralityof second pinholes corresponding to said first portion, and diffractedlight of said predetermined order that passes through said secondportion.
 23. An apparatus according to claim 16, further including afringe scanning means for moving said diffraction grating so as toperform fringe scanning.
 24. An apparatus for measuring wavefrontaberration of an optical system having an object plane and an imageplane, comprising: a. a light source for supplying coherent light; b. abeam splitter that splits said coherent light into first and secondlight beams each having an associated optical path length; c. a firstpinhole member, arranged in one of the object plane and image plane,that uses said first light beam to generate a first spherical wave; d. apinhole mirror having a plurality of apertures arrayed two-dimensionallyin the opposite one of the object plane and the image plane where saidfirst pinhole member is arranged, that transmits said second beam, and areflective portion that reflects said first light beam from said opticalsystem; and e. wherein the wavefront aberration of the optical system iscalculated based on interference fringes generated by interferencearising from a second spherical wave generated by said plurality ofapertures of said pinhole mirror based on said second light beam, andsaid first light beam reflected by said reflective portion of saidpinhole mirror.
 25. An apparatus according to claim 24, furtherincluding a fringe scanning means that changes at least one of saidoptical path lengths of said first and second light beams.
 26. Anapparatus for measuring wavefront aberration of an optical system havingan image plane and an object plane, comprising: a. a laser plasma X-raylight source; b. a first pinhole member provided with a first pinholegroup comprising a plurality of first pinholes that generates aplurality of first spherical waves based on said light; c. a secondpinhole member provided with a second pinhole group comprising aplurality of second pinholes arranged at the imaging position where saidfirst pinhole member is imaged by the optical system; d. a diffractiongrating arranged in the optical path between said first and secondpinhole members, and arranged so that zeroeth order diffracted lightpassing through said first pinhole group reaches said second pinholegroup. e. diffracted light selection means for selectively transmittingdiffracted light of a predetermined order from among diffracted light offirst order and higher order from said diffraction grating; and f. adetector for detecting interference fringes obtained by interference ofa second spherical wave generated by said zeroeth order diffracted lightpassing through said second pinhole group, and said diffracted light ofsaid predetermined order passing through said diffracted light selectionmeans.
 27. An apparatus according to claim 26, wherein: a. said firstpinhole member includes a plurality of first slit groups; b. said secondpinhole member includes a plurality of said second pinhole groupsarranged corresponding to the imaging position where said plurality offirst pinhole groups is imaged by the optical system; and c. saiddiffracted light selection means is a plate having a plurality ofapertures for selectively transmitting said plurality of diffractedlight of a predetermined order generated by passing through saiddiffraction grating a plurality of light beams proceeding to a pluralityof imaging positions.
 28. An apparatus according to claim 27, furtherincluding: a. first selective illumination means for selectivelyilluminating a first portion of said plurality of first pinholes of saidfirst pinhole member; and b. selective light receiving means forselectively receiving said second ideal spherical wavefront from asecond portion of said plurality of second pinholes corresponding tosaid first portion, and diffracted light of said predetermined orderthat passes through said second portion.
 29. An apparatus according toclaim 27, further including a second selective illumination means forselectively illuminating said portion of said second pinhole groups fromamong said plurality of second pinhole groups.
 30. An apparatus formeasuring wavefront aberration in an optical system having an imageplane and an object plane, comprising: a. a laser plasma X-ray lightsource capable of generating X-ray light; b. a first slit memberprovided with a first slit group comprising a plurality of first slitsthat generates a plurality of first one-dimensional spherical wavesbased on said X-ray light from said light source; c. a second slitmember provided with a second slit group comprising a plurality ofsecond slits arranged at an imaging position where said first slitmember is imaged by the optical system; d. a diffraction gratingarranged in the optical path between said first and second slit members,and arranged so that a zeroeth order diffracted light of said X-raylight passing through said first slit group reaches said second slitgroup; e. diffracted light selection means for selectively transmittingdiffracted X-ray light of non-zero order; and f. a detector that detectsinterference fringes arising from interference between a secondone-dimensional spherical wave generated when said zeroeth order X-raylight passes through said second slit group, and of said non-zeroethorder diffracted X-ray light passing through said diffracted lightselection means.
 31. An apparatus according to claim 30, wherein: a.said first slit member includes a plurality of said first slit groups;b. said second slit member includes a plurality of second slit groupsarranged corresponding to a plurality of imagine positions where saidplurality of first slit groups is imaged by the optical system; and c.said diffracted light selection means has a plurality of apertures forselectively transmitting a plurality of diffracted X-ray light generatedby passing through said diffraction grating a plurality of light beamsproceeding to said plurality of imaging positions.
 32. An apparatusaccording to claim 31, further including: a. first selectiveillumination means for selectively illuminating a first portion of saidplurality of first slit groups; and b. selective light receiving meansfor selectively receiving said second one-dimensional sphericalwavefront from a second portion of said plurality of second slit groupscorresponding to said first portion, and diffracted light of saidpredetermined order that passes through said second portion.
 33. Anapparatus according to claim 32, further including a second selectiveillumination means for selectively illuminating said portion of saidsecond slit from among said plurality of second slit groups.
 34. Anapparatus for measuring wavefront aberration in an optical system,comprising: a. a laser plasma X-ray light source for generating X-raylight; b. a slit member provided with a first slit group comprising aplurality of slits that generates a plurality of one-dimensionalspherical waves based on said light from said light source; c. adiffraction grating arranged in the optical path between said slitmember and an imaging position where said slit member is imaged by theoptical system, and that generated diffracted light from light passingtherethrough; d. diffracted light selection means for selectivelytransmitting diffracted light of a predetermined order and diffractedlight of an order different than said predetermined order; and e. adetector arranged so as to detect interference fringes from interferencebetween said diffracted light of a predetermined order and saiddiffracted light of an order different than said predetermined order toallow the calculation of the wavefront aberration from said interferencefringes.
 35. An apparatus for measuring wavefront aberration in anoptical system having an incident-side numerical aperture NA, an objectplane and an image plane, the apparatus comprising: a. an X-ray lightsource for generating X-ray light having a wavelength λ; b. a firstpinhole plate having an aperture smaller than 0.6 λ/NA arranged at theobject plane; c. a Hartmann plate arranged between said first pinholeplate and the image plane, said Hartmann plate having a plurality ofapertures; d. a detector arranged adjacent said image plane so as todetect a position of a plurality of ray groups passing through saidplurality of apertures of said Hartmann plate; and e. wherein thewavefront aberration is calculated based on said position of saidplurality of ray groups that drive on said detector.
 36. An apparatusaccording to claim 35, wherein said Hartmann plate is arranged betweenthe optical system and the image plane.
 37. An apparatus according toclaim 36, further including: a. first selective illumination means forselectively illuminating a portion of slit groups from among saidplurality of slit groups; b. detector position changing means thatchanges the detection position of said detector so as to detect said raygroup based on light passing through said portion of slit groups,wherein said slit member is provided with a plurality of said slitgroups.
 38. An interferometer calibration method for measuring a surfaceshape of an optical element of an optical system, the method comprisingthe steps of: a. interferometrically measuring the surface shape of theoptical element to obtain a surface shape measurement value; b.assembling the optical system by including the optical element in theoptical system; c. measuring a wavefront aberration of the opticalsystem; d. separating said wavefront aberration into a componentcorresponding to positional error of the surface shape and a componentcorresponding to surface shape error; e. correcting said positionalerror component and calculating said surface shape error component; andf. correcting said surface shape measurement value using said surfaceshape error component as calculated in said step e.